Production of optical pulses at a desired wavelength utilizing higher-order-mode (HOM) fiber

ABSTRACT

An apparatus and method for producing optical pulses of a desired wavelength utilizes a section of higher-order-mode (HOM) fiber to receive input optical pulses at a first wavelength, and thereafter produce output optical pulses at the desired wavelength through soliton self-frequency shifting (SSFS) or Cherenkov radiation. The HOM fiber is configured to exhibit a large positive dispersion and effective area at wavelengths less than 1300 nm.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.12/288,206, filed Oct. 17, 2008 now U.S. Pat. No. 8,126,299, which is acontinuation of U.S. Ser. No. 11/977,918, filed Oct. 26, 2007 nowabandoned, which claims the benefit of U.S. Provisional Applications60/863,082, filed Oct. 26, 2006, and 60/896,357, filed Mar. 22, 2007,both provisional applications herein incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to the production of optical pulses at adesired wavelength using higher-order-mode fibers and, more particular,to the utilization of HOM fiber with a positive dispersion and largeeffective area sufficient to generate high energy, short pulses atwavelengths below 1300 nm, considered useful for numerous applications.

BACKGROUND OF THE INVENTION

Mode-locked femtosecond fiber lasers at 1030 nm and 1550 nm have beenimproving significantly in the last several years, particularly withrespect to the achievable output pulse energy (increasing from 1 to ˜10nJ). Even higher pulse energy can be achieved in femtosecond fibersources based on fiber chirped pulse amplification. However, femtosecondfiber sources, including lasers, have seen only limited applications inmultiphoton imaging. The main reason is that they offer very limitedwavelength tunability (tens of nanometer at best), severely restrictingthe applicability of these lasers, making them only suitable for somespecial purposes. In addition, existing femtosecond fiber sources athigh pulse energy (>1 nJ) are not truly “all fiber,” i.e., the output isnot delivered through a single mode optical fiber. Thus, additionalsetup (typically involving free-space optics) must be used to deliverthe pulses to imaging apparatus, partially negating the advantages ofthe fiber source.

Reports have demonstrated the possibility of propagating femtosecond IRpulses through a large core optical fiber at intensities high enough (˜1nJ) for multiphoton imaging. In addition, a special HOM fiber that iscapable of delivering energetic femtosecond pulses (˜1 nJ) has beendemonstrated. However, both of these fibers have normal dispersion, andboth require a free-space grating pair for dispersion compensation. Notonly is such a grating pair lossy and complicated to align, it needscareful adjustment for varying fiber length, output wavelength, andoutput pulse energy, and falls short of the requirement for mostbiomedical research labs and future clinical applications.

Femtosecond fiber sources are highly robust and cost effective. However,energetic femtosecond fiber sources at 1300 nm prove to be verydifficult to obtain because of the characteristics of the fluoride fiberbased gain medium. Such difficulties are exemplified by the hugeperformance and cost gap between optical amplifiers (essentially laserswithout the cavity mirrors) at these wavelengths. For example, fiberamplifiers at 1300 nm are expensive (˜$31 k) and have limited outputpower (˜60 mW) and narrow spectral bandwidth (˜20 nm). This is in sharpcontrast to fiber amplifiers at 1030 nm and 1550 nm, where high poweramplifiers (˜2 W) will only cost ˜$25 k and has a spectral bandwidth of˜40 nm. Thus, the route that follows the development of fiber sources at1030 and 1550 nm is unlikely to be productive at 1300 nm. An alternativeapproach must be taken in order to create a femtosecond fiber source at1300 nm that has a comparable performance and cost to that of the fibersources at 1030 and 1550 nm.

Higher-order-mode (HOM) fiber has attracted significant interestrecently, due to the freedom it provides to design unique dispersioncharacteristics in all-solid (i.e., non-“holey”) silica fiber.

The ‘wavelength tunability’ of femtosecond optical sources has beenextensively studied within the phenomenon of soliton self-frequencyshift (SSFS), in which Raman self-pumping continuously transfers energyfrom higher to lower frequencies within an optical fiber. SSFS has beenexploited over the last decade in order to fabricate widelyfrequency-tunable, femtosecond pulse sources with fiber delivery. Sinceanomalous (positive) dispersion (β₂<0 or D>0) is required for thegeneration and maintenance of solitons, early sources that made use ofSSFS for wavelength tuning were restricted to wavelength regimes>1300nm, where conventional silica fibers naturally exhibit positivedispersion.

In addition, Cherenkov radiation has been demonstrated inmicrostructured fibers pumped near their zero-dispersion wavelength. Ingeneral, an ideal soliton requires a perfect balance between dispersionand nonlinearity so that energy becomes endlessly confined to a discretepacket—both spectrally and temporally. When perturbations areintroduced, this stable solution breaks down, allowing the transfer ofenergy between the soliton and the disturbance. Such energy transferoccurs most efficiently in fibers for solitons near the zero-dispersionwavelength. The spectral regime to which energy couples most efficientlyhas been dubbed “Cherenkov radiation” due to an analogous phase matchingcondition in particle physics. The phenomenon of Cherenkov radiation infibers is often associated with SSFS as it allows a convenient mechanismfor more efficient energy transfer between the soliton and the Cherenkovband. In particular, when the third-order dispersion is negative, SSFSwill shift the center frequency of the soliton toward thezero-dispersion wavelength, resulting in efficient energy transfer intothe Cherenkov radiation in the normal dispersion regime. The problem oftunability remains an issue for these arrangements capable of creatingCherenkov radiation.

The recent development of index-guided photonic crystal fibers (PCF) andair-core photonic band-gap fibers (PBGF) have relaxed this tunabilityrequirement somewhat, with the ability to design large positivewaveguide dispersion and therefore large positive net dispersion inoptical fibers at nearly any desired wavelength. This development hasallowed for a number of demonstrations of tunable SSFS sourcessupporting input wavelengths as low as 800 nm in the anomalousdispersion regime.

Unfortunately, the pulse energy required to support stable Raman-shiftedsolitons below 1300 nm in index-guided PCFs and air-core PBGFs is eitheron the very low side, a fraction of a nJ for silica-core PCFs, or on thevery high side, greater than 100 nJ (requiring an input from anamplified optical system) for air-core PBFGs. The low-energy limit isdue to high nonlinearity in the PCF. In order to generate large positivewaveguide dispersion to overcome the negative dispersion of thematerial, the effective area of the fiber core must be reduced. Forpositive total dispersion at wavelengths less than 1300 nm, thiscorresponds to an effective area, A_(eff), of 2-5 μm², approximately anorder of magnitude less than conventional single mode fiber (SMF). Thehigh-energy limit is due to low nonlinearity in the air-core PBGF wherethe nonlinear index, n₂, of air is roughly 1000 times less than that ofsilica. These extreme ends of nonlinearity dictate the required pulseenergy (U) for soliton propagation, which scales as U□D·A_(eff)/n₂. Infact, most microstructure fibers and tapered fibers with positivedispersion are intentionally designed to demonstrate nonlinear opticaleffects at the lowest possible pulse energy, while air-core PBGFs areoften used for applications that require linear propagation, such aspulse delivery.

There are a number of biomedical applications that require femtosecondsources. Although applications requiring a large spectral bandwidth(such as optical coherency tomography) can also be performed usingincoherent sources such as superluminscent diodes, techniques based onnonlinear optical effects, such as multiphoton microscopy and endoscopy,almost universally require the high peak power generated by afemtosecond source.

Molecular two-photon excitation (2PE) was theoretically predicted byMaria Goppert-Mayer in 1931. The first experimental demonstration oftwo-photon absorption, however, came nearly 30 years later, after thetechnological breakthrough of the invention of the ruby laser in 1960.It was almost another 30 years before the practical application of 2PEfor biological imaging was demonstrated at Cornell University in 1990.Once again, this new development was propelled in large part by therapid technological advances in mode-locked femtosecond lasers. Sincethen, two-photon laser scanning microscopy has been increasingly appliedto cell biology and neurosciences. A number of variations, includingthree-photon excitation (3PE), second and third harmonic generationimaging, near-field enhanced multiphoton excitation and multiphotonendoscopic imaging, have emerged and further broadened the field, whichis currently known as multiphoton microscopy (MPM). Today, MPM is anindispensable tool in biological imaging. Like any nonlinear process,however, multiphoton excitation requires high peak intensities,typically 0.1 to 1 TW/cm² (TW=10¹² W). Besides tight spatial focusing,MPM typically requires pulsed excitation sources to provide additionaltemporal “focusing” so that efficient multiphoton excitation can beobtained at low average power. For example, a femtosecond laser with100-fs pulse width (τ) at 100 MHz pulse repetition rate (f) will enhancethe excitation probability of 2PE by a factor of 10⁵, i.e., the inverseof the duty cycle (fτ). The development of multiphoton imaging dependscritically on ultrafast technologies, particularly pulsed excitationsource.

Endoscopes play an important role in medical diagnostics by making itpossible to visualize tissue at remote internal sites in a minimallyinvasive fashion. The most common form employs an imaging fiber bundleto provide high quality white light reflection imaging. Laser scanningconfocal reflection and fluorescence endoscopes also exist and canprovide 3D cellular resolution in tissues. Confocal endoscopes are nowbecoming available commercially (Optiscan Ltd, Australia, Lucid Inc,Rochester) and are being applied in a number of clinical trials forcancer diagnosis. Multiphoton excitation based endoscopes has attractedsignificant attention recently. There were a number of advances,including fiber delivery of excitation pulses, miniature scanners,double clad fibers for efficient signal collections, etc. Thus, justlike MPM has proven to be a powerful tool in biological imaging,multiphoton endoscopes have great potentials to improve the capabilityof the existing laser-scanning optical endoscopes. It is quite obviousthat a compact, fully electronically controlled, femtosecond systemseamlessly integrated with fiber optic delivery is essential formultiphoton endoscopy in medical diagnostics, particularly to biomedicalexperts who are not trained in lasers and optics.

Perhaps the most promising and successful area in biomedical imagingthat showcases the unique advantage of multiphoton excitation is imagingdeep into scattering tissues. In the past 5 to 10 years, MPM has greatlyimproved the penetration depth of optical imaging and proven to be wellsuited for a variety of imaging applications deep within intact orsemi-intact tissues, such as demonstrated in the studies of neuronalactivity and anatomy, developing embryos, and tissue morphology andpathology. When compared to one-photon confocal microscopy, a factor of2 to 3 improvement in penetration depth is obtained in MPM. Nonetheless,despite the heroic effort of employing energetic pulses (˜μJ/pulse)produced by a regenerative amplifier, MPM has so far been restricted toless than 1 mm in penetration depth. One promising direction for imagingdeep into scattering tissue is to use longer excitation wavelength.Although there is little data for tissue scattering beyond 1.1 μm, theavailable data at shorter wavelengths clearly indicates the generaltrend that tissue scattering reduces as one uses longer excitationwavelength. Longer excitation wavelengths are an obvious choice forimaging deep into scattering tissues.

A concern for longer wavelength excitation is water absorption, which istypically the dominant contribution to tissue absorption at near IR.However, a careful examination showed that water absorption at 1270 nmis only approximately twice as high as that between 960 and 1000 nm, aspectral region where multiphoton excitation and imaging has beenroutinely performed in the past. Although the “diagnostic andtherapeutic window,” which is in between the absorption regions of theintrinsic molecules and water, extends all the way to ˜1300 nm, previousinvestigations involving multiphoton imaging are almost exclusivelycarried out within the near IR spectral window of ˜700 to 1100 nm,constrained mostly by the availability of the excitation source.Currently, there are only two femtosecond sources at the spectral windowof 1200 to 1300 nm, the Cr:Forsterite laser and the optical parametricoscillator (OPO) pumped by a femtosecond Ti:Sapphire (Ti:S) laser. Interms of robustness and easy operation, both sources rank significantlybelow the Ti:S laser. Thus, the development of a reliable fiber sourcetunable from 1030 to 1280 nm will open up new opportunities forbiomedical imaging, particularly for applications requiring deep tissuepenetration.

Shortly after the inception of MPM, mode-locked solid state femtosecondlasers, most commonly the Ti:S lasers, have emerged as the favoriteexcitation sources to dominate the MPM field today. When compared toearlier ultrafast lasers, e.g., ultrafast dye lasers, the Ti:S lasersare highly robust and flexible. The concurrent development of themode-locked Ti:S lasers was perhaps the biggest gift for MPM and enabledMPM to rapidly become a valuable instrument for biological research.Nonetheless, the cost, complexity, and the limited potential forintegration of the bulk solid state lasers have hampered the widespreadapplications of MPM in biological research. The fact that adisproportionate number of MPM systems are located in physics andengineering departments, instead of the more biologically orientedinstitutions, reflects at least in part the practical limitations of thefemtosecond pulsed source. Obviously, the requirement of a robust, fiberdelivered, and cheap source is even more urgent for multiphotonendoscopy in a clinical environment.

For these reasons, previous work using SSFS below 1300 nm was performedat soliton energies either too low or too high (by at least an order ofmagnitude) for many practical applications, such as multiphoton imaging,where bulk solid state lasers are currently the mainstay for theexcitation source.

The present invention is directed to overcoming these and otherdeficiencies in the state of the art.

SUMMARY OF THE INVENTION

The present invention relates to a higher-order-mode (HOM) fiber moduleoperable to generate energetic, short output pulses of light atwavelengths amenable to various applications, while also providing adegree of wavelength tunability. In particular, the inventive HOM moduleincludes a section of HOM fiber with anomalous (positive) dispersion anda large effective area, characteristics that create a solitonself-frequency shift sufficient to move an incoming stream of pulses atone wavelength to a stream of pulses at a second, desired wavelengthassociated with a specific application. These dispersion characteristicshave also been found to allow for the creation of soliton Cherenkovradiation at wavelengths below 1300 nm, with usable energy in the rangeof 1-10 nJ.

Additionally, the HOM fiber module of the present invention provides theability to compensate the dispersion of an optical pulse that is chirpedat its input (chirp of no less than 5.25 fs/nm). It has been found thatthe HOM module provides a sufficient amount of dispersion to provide atransform-limited pulse at a predetermined location within the HOM fibersuch that the pulse undergoes frequency shift by either of the SSFS orCherenkov effects described above.

In accordance with the present invention, the HOM module comprises aninput mode converter (for converting from the conventional LP₀₁ mode toa higher-order mode), a section of HOM fiber coupled to the input modeconverter for generating the desired self-frequency shift to a desiredoutput wavelength, and (when necessary) an output mode converter (forconverting the wavelength-shifted pulses back to the conventional LP₀₁mode or any other desired spatial profile).

In one embodiment, in-fiber long period gratings (LPGs) are used for theinput and output mode converters, thus minimizing the amount of opticalloss present at the junction between the mode converters and the HOMfiber.

The HOM fiber portion of the module is configured in one embodiment toinclude a wide, low index ring cladding area, separated from a highindex core region by a trench. The index values and dimensions of thering, trench and core are selected to provide the desired amount ofanomalous dispersion and size of the effective area. One set ofacceptable values for use in accordance with the present invention is adispersion on the order of +60 ps/nm-km and an effective area ofapproximately 44 μm². Another set of acceptable values are defined bythe wavelength range within which the dispersion is anomalous(positive), this range being between 10 and 300 nm. Yet another set ofacceptable values are defined by the maximum achievable dispersion inthe wavelength range of interest, this value ranging from 0 to +3000ps/nm-km. With respect to the effective area, acceptable values ofA_(eff) for the purposes of the present invention range from about 5 to4000 μm².

The present invention also relates to a method of producing outputoptical pulses having a desired wavelength. The method includesgenerating input optical pulses and delivering the input pulses to anHOM fiber module to alter the wavelength of the input optical pulsesfrom the first wavelength to the second, desired wavelength by solitonself-frequency shifting (SSFS) within the HOM fiber module.

In one embodiment, the method can further include converting the spatialmode of the input signal into a higher-order mode at the input of theHOM fiber module, and thereafter reconverting the output of the HOMfiber module back to the original spatial mode or to any other desiredmode profile.

It is an advantage of the present invention that the HOM module iscapable of achieving these characteristics at wavelengths below 1300 nm,heretofore not accomplished in an all-silica (non-holey) fiber.

Further, the HOM module of the present invention is designed such thatthe difference between the effective index n_(eff) of the mode in whichsignal propagation is desired is separated from that of any other guidedmode of the fiber by greater than 10⁻⁵, thus providing for enhancedmodal stability of the signal.

In one embodiment, the input comprises a single mode fiber (SMF) splicedto the HOM fiber before mode conversion, with the properties of thesplice ensuring that signal propagation in the HOM fiber occurspredominantly in the LP₀₁ mode, further enhancing modal stability forthe signal.

In one embodiment, the HOM fiber module includes an HOM fiber. SuitableHOM fibers can include, without limitation, a solid silica-based fiber.In another embodiment, the HOM fiber module includes an HOM fiber and atleast one mode converter. The at least one mode converter can beconnectedly disposed between the optical pulse source and the HOM fiber.The HOM fiber module can also include an HOM fiber, a mode converterconnectedly disposed between the optical pulse source and the HOM fiber,and also a second mode converter terminally connected to the HOM fiber.Suitable mode converters that can be used in the present invention arewell known in the art, and can include, for example, a long periodgrating (LPG). Successive HOM modules, each with differentcharacteristics, can be concatenated together, forming a cascadedarrangement configured to generate SSFS and/or Cherenkov radiation atmultiple wavelengths.

Suitable optical pulse sources that can be used in the present inventioncan include, without limitation, mode-locked lasers and chirped pulseamplification (CPA) systems. More particularly, the mode-locked lasercan be a mode-locked fiber laser, and the CPA system can be a fiber CPAsystem. The optical pulse source in the present invention can includethose that generate input optical pulses having various pulse energies.In one embodiment, the optical pulse source generates a pulse energy ofat least 1.0 nanojoules (nJ). In another embodiment, the optical pulsesource generates input optical pulses having a pulse energy of betweenabout 1.0 nJ and about 100 nJ.

The optical pulse source can also be one that generates input opticalpulses such that the first wavelength is a wavelength within thetransparent region of a silica-based fiber. In one embodiment, theoptical pulse source is one that generates a first wavelength below 1300nanometers (nm). In another embodiment, the optical pulse source is onethat generates a first wavelength between the range of about 300 nm andabout 1300 nm.

The optical pulse source used in the present invention can also be onethat generates input optical pulses having a subpicosecond pulse width.

Suitable HOM fiber modules that can be used in the present invention caninclude, without limitation, HOM fiber modules that produce outputoptical pulses having a pulse energy of at least 1.0 nJ. Suitable HOMfiber modules can also be those that produce output optical pulses suchthat the desired wavelength that is below 1300 nm. In anotherembodiment, the HOM fiber module produces an output optical pulse havinga desired wavelength between the range of about 300 nm and about 1300nm. The HOM fiber module can also be such that it produces outputoptical pulses having a subpicosecond pulse width.

The apparatus of the present invention can further include a powercontrol system connectedly disposed between the optical pulse source andthe HOM fiber module. The power control system for use in the presentinvention can be one that achieves subnanosecond power tuning of thefirst wavelength. Suitable power control systems can include, withoutlimitation, a lithium niobate (LiNbO₃) intensity modulator device.

The apparatus of the present invention can further include a single-modefiber (SMF) connectedly disposed between the optical pulse source andthe HOM fiber module.

The apparatus of the present invention can be used in a variety ofapplications where optical pulses of a desired wavelength are needed.For example, the apparatus can be effective in producing output opticalpulses that can penetrate animal or plant tissue at a penetration depthof at least 0.1 millimeters (mm).

The apparatus of the present invention can further be such that the HOMfiber module is terminally associated with medical diagnostic tools suchas an endoscope or an optical biopsy needle.

The apparatus of the present invention can further be functionallyassociated with a multiphoton microscope system.

The apparatus of the present invention can also further be functionallyassociated with a multiphoton imaging system.

The present invention also relates to a method of producing opticalpulses having a desired wavelength. This method includes generatinginput optical pulses using an optical pulse source, where the inputoptical pulses have a first wavelength and a first spatial mode. Theinput optical pulses are delivered into an HOM fiber module to alter thewavelength of the input optical pulses from the first wavelength to adesired wavelength by soliton self-frequency shift (SSFS) within the HOMfiber module, thereby producing output optical pulses having the desiredwavelength.

The method of the present invention can involve the use of the apparatusdescribed herein as well as the various aspects and components of theapparatus (e.g., the optical pulse source and the HOM fiber module)described herein.

In one embodiment, the method can further include converting the firstspatial mode of the input optical pulses into a second spatial modeprior to delivering the input optical pulses into the HOM fiber so thatthe output optical pulses have the second spatial mode, where the firstspatial mode and the second spatial mode are different modes. Thismethod can further include reconverting the second spatial mode of theoutput optical pulses back to the first spatial mode.

In another embodiment, the method can further include tuning the firstwavelength of the input optical pulses to an intermediate wavelengthprior to delivering the input optical pulses into the HOM fiber. Thetuning can include, without limitation, power tuning. Such power tuningcan include varying the power of the input optical pulses so as to varythe desired wavelength. In one embodiment, the power tuning can includesubnanosecond power tuning using a power control system connectedlydisposed between the optical pulse source and the HOM fiber module.Suitable power control systems can include, without limitation, alithium niobate intensity modulator device. In another embodiment, thetuning can be achieved by varying the length of the HOM fiber so as tovary the desired wavelength.

Described in more detail below is the concept of SSFS and Cherenkovradiation in optical fibers and more particularly in HOM fibers.

Other and further aspects and embodiments of the present invention willbecome apparent during the course of the following discussion and byreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings,

FIG. 1( a): Total dispersion for propagation in the LP₀₂ mode;

FIG. 1( b): Experimental near-field image of the LP₀₂ mode withA_(eff)=44 μm²;

FIG. 1( c): Experimental setup used to couple light through the HOMfiber module and generate soliton self-frequency shifting;

FIG. 2( a): Soliton self-frequency shifted spectra corresponding todifferent input pulse energies into the HOM fiber, with all traces takenat 4.0 nm resolution bandwidth (RBW) and input pulse energy noted oneach trace. Power conversion efficiency is 57% for 1.39 nJ input;

FIG. 2( b): High resolution trace of the initial spectrum; 0.1 nm RBW;

FIG. 2( c): High resolution trace of the shifted soliton for 1.63 nJinput into the HOM; 0.1 nm RBW;

FIG. 2( d): Soliton self-frequency shifted spectra calculated fromsimulation;

FIG. 3: Second-order interferometric autocorrelation trace of HOM outputfor 1.39 nJ input pulses. Autocorrelation FWHM measured to be 92 fscorresponding to a deconvolved pulse width of 49 fs;

FIG. 4( a): Experimental setup used to couple light through the HOMfiber module and measure the Cherenkov pulse generated in the HOM fiber;

FIG. 4( b): Total dispersion for propagation in the LP₀₂ mode;

FIG. 4( c): Experimental near-field image of the LP₀₂ mode withA_(eff)=44 μm²;

FIG. 5( a). Optical spectrum at output of HOM fiber module of FIG. 4from experiment;

FIG. 5( b). Optical spectrum at output of HOM fiber module of FIG. 4from simulation, where insets show input spectra on a 30 nm window;

FIG. 6. Cherenkov output pulse energy as a function of input pulseenergy;

FIG. 7. Intensity autocorrelation traces of Cherenkov pulse, (a) at theoutput of the HOM fiber module, without dispersion compensation, and (b)dispersion compensated pulse, with interferometric autocorrelationtraces shown as insets, this corresponds to pulsewidths of 465 fs and106 fs in (a) and (b), respectively;

FIG. 8 shows an absorption coefficient of water as a function ofwavelength;

FIG. 9 is a schematic drawing of one embodiment of an all fiber,wavelength tunable, energetic, femtosecond source;

FIG. 10( a) is an output spectrum and FIG. 10( b) is a second-orderautocorrelation measurement of the pulse width (˜300 fs) of a commercialfiber source (Uranus 001, PolarOnyx Inc.);

FIG. 10( c) is a photograph of the fiber source. The lateral dimensionof the source is about one foot. Data and photograph courtesy ofPolarOnyx Inc.;

FIG. 11 shows an output of self-similar laser. Left: theoreticalspectrum, output pulse, and equi-intensity contours of the pulse as ittraverses the laser. Right: experimental spectrum (on logarithmic andlinear scales), and measured autocorrelations of the pulse directly fromthe laser (broad pulse) and after dechirping (short pulse);

FIG. 12 shows a comparison of modal behavior between conventional LP01(single mode fiber, top—schematic) and LP02 (bottom—simulated) modes.FIG. 12( a): Near-field images. FIG. 12( b): Mode profiles at variouswavelengths. Conventional mode transitions from high to low index;designed HOM shows opposite evolution. Grey background denotes indexprofile of the fiber. FIG. 12( c): Resultant total dispersion(D_(total), solid), also shown are silica material dispersion (D_(m),dashed) and zero-dispersion line (dotted), where arrows showcontribution of waveguide dispersion (D_(w) to total dispersion;

FIG. 13 is an index profile of the HOM fiber.

FIG. 14 shows an experimentally measured near-field image LP02 mode withan effective area A_(eff)˜44 μm².

FIG. 15 is a refractive index profile for an HOM fiber of the presentinvention, indicating the set of six different parameters than may beadjusted to provide the desired positive dispersion and large effectivearea;

FIG. 16 is a graph of the simulated total dispersion vs. wavelengthcurves for a variety of profiles, forming by adjusting one or more ofthe parameters shown in FIG. 15;

FIG. 17 shows index vs. radial position of the designed and fabricatedfiber measured at several perform positions;

FIG. 18 illustrates an exemplary HOM module for converting an inputwavelength to a desired output wavelength in accordance with the presentinvention;

FIG. 19( a) is a plot of the bandwidth of the HOM module of FIG. 18;

FIG. 19( b): Comparison of the dispersions of the HOM fiber (solid) andthe conventional SMF (dashed). Also shown is the zero-dispersion line(dotted);

FIG. 19( c): Comparison of the dispersions over the narrow range of 1040to 1120 nm;

FIG. 20 is a demonstration of SSFS in a tapered PCF (inset in b). (a)Output spectra at different values of output soliton power. (b) Measuredwavelength shift vs. input power;

FIG. 21 shows results of SSFS in a PCF. A pulse at 1030 nm is shifted tobeyond 1300 nm in this example. Result of numerical simulation is shownfor comparison;

FIG. 22( a) is a photo of the HOM fiber module for the demonstration ofCherenkov radiation;

FIG. 22( b) is an associated block diagram showing the placement of theHOM fiber module in the overall system. The splice protector alsoprotects the in-fiber FPG mode converter;

FIG. 23( a): Soliton self-frequency shifted and Cherenkov spectracorresponding to different input pulse energies into the HOM fiber ofFIG. 22( b);

FIG. 23( b): High resolution trace of the initial spectrum;

FIG. 23( c): High resolution trace of the shifted soliton for 1.63 nJinput into the HOM;

FIG. 23( d): Soliton self-frequency shifted and Cherenkov spectracalculated from simulation;

FIG. 24( a) is a measured spectral trace showing the Cherenkov radiationat the maximum achievable 2.0-nJ input power into the HOM fiber in thesystem of FIG. 22( b). Inset is the measured second-orderinterferometric autocorrelation trace (total delay of 2 ps) of thefiltered Cherenkov radiation, corresponding to a deconvolved pulse widthof approximately 100 fs (FWHM);

FIG. 24( b): Results from numerical simulation using the experimentalparameters from FIG. 22( b). Inset is the simulated second-orderinterferometric autocorrelation trace of the filtered Cherenkovradiation. The input center wavelength is at 1064 nm. The spectral peaksin the middle (˜1200 nm) are the Raman shifted solitons;

FIG. 25( a): Designed dispersion optimized for a 775-nm input;

FIG. 25( b): Dispersion tuning range, illustrating a tuning range ofabout 50 nm for the ZDW of the HOM fiber, for a peak wavelength shiftfor 775-nm and 1030-nm input signals;

FIG. 26( a): Designed dispersion (D, dashed line) vs. wavelength forconverting 1.03 μm to 1.27 μm via Cherenkov radiation. The dispersion ofthe existing HOM fiber (solid line) is also indicated;

FIGS. 26( b) and 26(c): Output intensity (b) and spectrum (c) afterpropagating through 0.25-meter of HOM fiber at an input pulse energy of4.2 nJ. For comparison, the input spectrum (scaled down by a factor of5) and pulse (dashed lines) are also shown in FIG. 26( b). The spectralfilter bandwidth is indicated in FIG. 26( c).

FIG. 27( a): Designed dispersion (D) vs. wavelength curves forconverting 1.03 μm to 1.27 μm via Cherenkov radiation for high pulseenergy (input pulse energy of 22.4 nJ);

FIGS. 27( b) and 27(c): Output intensity (b) and spectrum (c) afterpropagating through a 3-cm HOM fiber at an input energy of 22.4 nJ. Forcomparison, the input spectrum (scaled down by a factor of 5) and pulse(dashed line) are also shown in FIG. 27( b). The output pulse width andenergy are also indicated in FIG. 27( c);

FIG. 28 shows output spectra (a) at various input pulse energies for a1-meter HOM fiber and (b) at various propagation distance (z) in the HOMfiber (i.e., HOM fiber length) for an input pulse energy of 1.5 nJ. Forcomparison, the input spectrum is also shown;

FIG. 29 shows two-photon excitation spectra of fluorophores. Datarepresent two-photon action cross-section, i.e., the product of thefluorescence emission quantum efficiencies and the two-photon absorptioncross sections. 1 GM≡10⁻⁵⁰ cm⁴ s/photon, where the spectra are excitedwith linearly polarized light using a Ti:S pumped OPO;

FIG. 30 is a temporal pulse evolution in an HOM fiber module at variouspropagation distances (z) with a 2.6-nJ chirped input pulse: (a) z=0;(b) z=3 m; (c) z=3.9 m; and (d) z=6.11 m. Insert in (d) is the zoom-inversion of the soliton pulse, and the FWHM of the soliton is 44 fs;

FIG. 31: Output intensity at various propagation distances (z) at aninput pulse energy of 22.4 nJ, where no post dispersion compensation isemployed: (a) z=2 cm; (b) z=2.2 cm; and (c) z=2.5 cm. The pulse width(FWHM) and energy are also indicated;

FIG. 32 shows energy of self-similar pulses (up-triangles) obtained innumerical simulations of fiber laser, plotted versus net cavitydispersion. The down-triangles are the energies produced bystretched-pulse operation of the laser;

FIG. 33 is a schematic drawing of the proposed all fiber, wavelengthtunable, energetic, femtosecond source after full system integration;

FIG. 34: Schematic drawing of the proposed energetic fiber femtosecondsource after full system integration (bottom) and more detailedphotographs and device setup (top). Dispersion compensation using aprism pair is shown and is optional for high energy output. The modeprofiles of the fundamental and LP₀₂ modes are also shown;

FIG. 35 shows an instrument for multiphoton spectroscopy on cancertissues. The inset shows a schematic contour plot of theexcitation-emission matrix (EEM); and

FIG. 36 shows a two-photon excitation spectra (a) and emission spectra(b) of CFP and monomeric eGFP, two common genetically encodablefluorescent proteins. A system capable of switching the excitationwavelength of ms timescales (i.e., between forward and return scanlines) would be able to more cleanly separate the emissions.

DETAILED DESCRIPTION

Soliton self-frequency shifting (SSFS) is a well-known andwell-understood phenomenon. The concept of SSFS was first discovered ˜20years ago in fiber optic communications, and most of the pastexperiments on SSFS relates to telecom. Optical soliton pulses generallyexperience a continuous downshift of their carrier frequencies whenpropagating in a fiber with anomalous dispersion. This so-called solitonself-frequency shift originates from the intra-pulse stimulated Ramanscattering which transfers the short wavelength part of the pulsespectrum toward the long wavelength part (SSFS sometimes is also calledRaman soliton shift). Through the balancing of optical nonlinearity andfiber dispersion (i.e., soliton condition), the pulse maintains itstemporal and spectral profiles as it shifts to the longer wavelengths.Although the physics of SSFS was well known for the last 20 years, itspractical application was limited because the use of conventional fibersfor generating wavelength-shifting solitons has major limitations.However, several new classes of optical fibers, such as photonic crystalfibers (PCF, sometimes also known as microstructure fiber) andsolid-core or air-core band gap fibers (BGF), has generated enormousexcitement in the last 5 years and greatly improved the feasibility ofSSFS. Indeed, there are a number of experimental demonstrations of SSFSin PCF and BGF. However, none of the previous work can generate solitonenergies that are of practical interest to biomedical research, i.e.,solitons with pulse energies between 1 to 10 nJ and at wavelengths below1300 nm. As we will elaborate below, the pulse energies produced inprevious works are either one or two orders of magnitude too small orseveral orders of magnitude too large.

Because material nonlinearity for silica glass is positive at therelevant spectral range, the fundamental condition to form an opticalsoliton in silica fiber is anomalous dispersion. In addition, theexistence of an optical soliton requires exact balance between fibernonlinearity and dispersion. Thus, the energy of an optical soliton(E_(s)) is determined by material nonlinearity and dispersion, andscales asE_(S)∝λ³·D·A_(eff)/n₂τ  (1),where n₂ is the nonlinear refractive index of the material, τ is thepulse width, D is the dispersion parameter, A_(eff) is the effectivemode field area, and λ is the wavelength. Although standard single modefibers (SMF) cannot achieve anomalous dispersion at λ<1280 nm, it wasrealized that the total dispersion (D) in a waveguide structure such asan optical fiber consists of contributions from the material (D_(m)),the waveguide (D_(w)), and the bandgap (in the case of BGF). Byappropriately engineering the contributions of the waveguide and/or thebandgap, it is possible to achieve anomalous dispersion (D>0) atvirtually any wavelength, thus, enabling soliton and SSFS at wavelengthsbelow 1280 nm. (It is worth noting that the dispersion parameter D isactually positive for anomalous dispersion.) Previously, there were twoapproaches to achieve anomalous dispersion, and therefore solitonpropagation and SSFS, at λ<1280 nm;

(1) Small-core PCF can achieve anomalous dispersion for wavelengths downto ˜550 nm. When the waveguide is tightly confining, with the air-silicaboundary defining the confinement layer, the waveguide dispersion(D_(w)) is akin to that of microwave waveguides with perfectlyreflecting walls. Hence, large positive waveguide dispersion may berealized by tightly-confined LP₀₁ (fundamental) modes in PCFs. However,the associated trade-off is with A_(eff), and designs that yielddispersion>+50 ps/mm/km in the wavelength ranges of 800 nm or 1030 nmtypically have A_(eff) of 2-5 μm². Because the soliton energy scaleswith the value of D*A_(eff), a small A_(eff) will severely limit thepulse energies that can be obtained with PCFs. For example, in oneexperiment using a special PCF structure, a soliton pulse energy of ˜20pJ was obtained at 800 nm, orders of magnitude smaller than practicalfor imaging. Indeed, most PCF structures are designed to demonstratenonlinear optical effects at the lowest possible pulse energy.

(2) Air-guided BGFs potentially can offer anomalous dispersion at anyguiding wavelength, but the extremely low nonlinearities in these fibers(the nonlinearity of air is ˜one thousand times smaller than silicaglass) make them impractical for a device that utilizes a nonlinearinteraction to achieve the frequency shift. In one demonstration, a MW(˜μJ pulse) optical amplifier was needed for observing SSFS inair-guiding fiber. Not only is such a high power unnecessary for mostbiomedical applications, the cost and complexity of the high poweramplifier also makes it completely impractical as a tool for biomedicalresearch.

Although SSFS provides a convenient mechanism for wavelength tuning of afixed wavelength fiber laser, previous works in SSFS were performed atsoliton energies either too low or too high (by at least an order ofmagnitude) for practical use. Thus, it is essential to invent a newfiber structure, with just the right amount of optical nonlinearity anddispersion (i.e., D·A_(eff)/n₂) in order to produce soliton pulses ofpractical utility for biomedical imaging.

An optical fiber generally propagates a number of spatial modes(electric field states). Because of modal dispersion and interference,however, only single mode fibers (i.e., fibers with only one propagatingmode) are of interest for applications such as high speed datatransmission and pulse delivery for imaging. It was realized, however,that a multimode fiber can propagate only one mode if two conditions aremet: (1) the input field is a pure single mode and (2) the couplingsbetween various modes during propagation are small. In the case that theone propagating mode is not the fundamental mode, the fiber is called aHOM fiber. HOM fibers first attracted attention in opticalcommunications nearly ten years ago. The main motivation was fordispersion compensation of high bit-rate optical communications. Theadvantage of HOM fibers is to provide another degree of freedom in thedesign space to achieve the desired dispersion characteristics. Therewere a number of devices invented using HOM fibers. In fact, dispersioncompensators based on HOM fibers have been commercially available forseveral years.

It has been realized that the design freedoms enabled by HOM fibers areexactly what is needed for achieving the desired soliton energy atwavelengths below 1300 nm for biomedical imaging: (1) A higher ordermode can achieve anomalous dispersion at wavelength below 1300 nm, acondition necessary for soliton and impossible to obtain in aconventional silica SMF. (2) A higher order mode typically has a muchlarger A_(eff) than that of PCF for achieving higher soliton energy. (3)The silica core of the HOM fiber retains just enough nonlinearity tomake SSFS feasible at practical energy level. (4) The all silica HOMfiber retains the low loss properties (for both transmission andbending) of a conventional SMF, and allows easy termination andsplicing. (5) A HOM fiber leverages the standard silica fibermanufacturing platform, which has been perfected over the course of 30years with enormous resources. Thus, an appropriately designed HUM fibercan provide the necessary characteristics desired for biomedicalimaging, and can be manufactured immediately with high reliability.

EXAMPLES

The Examples set forth below are for illustrative purposes only and arenot intended to limit, in any way, the scope of the present invention.

Example 1 Demonstration of Soliton Self-Frequency Shift Below 1300 nm inHigher-Order-Mode, Solid Silica-Based Fiber

Soliton-self frequency shift of more than 12% of the optical frequencywas demonstrated in a higher-order-mode (HUM) solid, silica-based fiberbelow 1300 nm. This new class of fiber shows great promise of supportingRaman-shifted solitons below 1300 nm in intermediate energy regimes of 1to 10 nJ that cannot be reached by index-guided photonic crystal fibersor air-core photonic band-gap fibers. By changing the input pulse energyof 200 fs pulses from 1.36 nJ to 1.63 nJ, clean Raman-shifted solitonswere observed between 1064 nm and 1200 nm with up to 57% powerconversion efficiency and compressed output pulse widths less than 50fs. Furthermore, due to the dispersion characteristics of the HOM fiber,red-shifted Cherenkov radiation in the normal dispersion regime forappropriately energetic input pulses was observed at energies of 1.45 nJand above.

The HOM fiber used in the experiments of this example was shown toexhibit large positive dispersion (˜60 ps/nm-km) below 1300 nm whilestill maintaining a relatively large effective area of 44 μm², ten timesthat of index-guided PCFs for similar dispersion characteristics.Through soliton shaping and higher-order soliton compression within theHOM fiber, clean 49 fs output pulses from 200 fs input pulses weregenerated.

FIG. 1( a) shows the dispersion curve for the LP₀₂ mode in the HOM fiberused in the experiment of the present example. To generate positivedispersion below 1300 nm while simultaneously maintaining a largeeffective area, light propagates solely in the LP₀₂ mode. Light iscoupled into the LP₀₂ mode using a low-loss long period grating (LPG).The index profile of the HOM fiber is made such that the mode becomesmore confined to the higher-index core with an increase in wavelength,resulting in net positive dispersion. FIG. 1( a) shows a dispersion of62.8 ps/nm-km at 1060 nm which is comparable to that of microstructuredfibers used previously for SSFS and exhibits two zero dispersionwavelengths at 908 nm and 1247 nm. The mode profile at the end face ofthe HOM fiber is shown in FIG. 1( b), demonstrating a clean higher-orderLP₀₂ mode and an effective area of 44 μm².

A schematic of the fiber-module 8 used for this experiment is shown inFIG. 1( c). Here light propagates in the fundamental mode through 12.5cm of standard single mode (Flexcore) fiber 10 before being coupled into1.0 m of the HOM fiber 12 with a 2.5 cm long period grating (LPG) 14,entirely contained within a fiber fusion-splicing sleeve. Light residesin the LP₀₁ mode for approximately half the length of the grating 14after which more than 99% is coupled into the LP₀₂ mode. The entiremodule 8 has a total loss of 0.14 dB which includes all splices, fiberloss, and mode conversion. It is also noted that the all-silica HOMfiber 12 leverages the standard silica fiber manufacturing platform andretains the low loss properties (for both transmission and bending) of aconventional SMF, allowing easy termination and splicing. Thecombination of the single mode fiber 10 and LPG 14 creates an inputsignal at the HOM 12 which exhibits a linear chirp of at least 5.25fs/nm.

The pump source 16 consisted of a fiber laser (Fianium FP1060-IS) whichdelivered a free space output of ˜200 fs pulses at a center wavelengthof 1064 nm and an 80 MHz repetition rate. A maximum power of 130 mW wasable to be coupled into the fiber module 8, corresponding to 1.63 nJinput pulses. The input pulse energy was varied from 1.36 nJ to 1.63 nJto obtain clean spectrally-shifted solitons with a maximum wavelengthshift of 136 nm (12% of the carrier wavelength), FIG. 2( a). Theoreticaltraces from numerical simulation for similar input pulse energy areplotted adjacent to the experimental data in FIG. 2( d). The split-stepFourier method was used in the simulation and included self-phasemodulation (SPM), stimulated Raman scattering (SRS), self-steepening,and dispersion up to fifth-order. The dispersion coefficients wereobtained by numerically fitting the experimental curve in FIG. 1( a) anda nonlinear parameter γ=2.2 W⁻¹ Km⁻¹ and a Raman response of T_(R)=5 fswere used in the simulation. The irregularly shaped spectrum of theinput source was also approximated (FIG. 2( b)) with an 8.5 nm, Gaussianshape corresponding to 200 fs Gaussian pulses. Though a more accuratedescription should include the full integral form of the nonlinearSchrödinger equation, the excellent qualitative match and reasonablequantitative match validates this approach.

57% power conversion from the input pulse spectrum to the red-shiftedsoliton was measured for the case of 1.39 nJ input pulses to achieve˜0.8 nJ output soliton pulses, FIG. 2( a). The correspondingsecond-order interferometric autocorrelation (FIG. 3) gives an outputpulse width of 49 fs, assuming a sech² pulse shape, showing a factor offour in pulse width reduction due to higher-order soliton compression(soliton order N=2.1) in the HOM fiber 12. The measured spectralbandwidth of 35 nm gives a time-bandwidth product of 0.386 which is 23%beyond that expected for a sech² pulse shape. It is believed that thediscrepancy is likely due to dispersion from ˜5 cm of glass (collimatingand focusing lenses 18) between the fiber output and the two-photondetector inside the autocorrelator 19. This explanation is supported bynumerical simulation which gives an output pulse width of 40 fs. Offurther note is the ripple-free, high resolution spectrum of the shiftedsoliton for 1.63 nJ input, FIG. 2( c). This is indicative of propagationexclusively in the LP₀₂ mode since multimode propagation would surfaceas spectral interference.

Finally, the appearance of Cherenkov radiation centered about 1350 nmfor 1.45 nJ and 1.63 nJ input pulse energies, FIG. 2( a). Here, as hasbeen demonstrated previously, Cherenkov radiation is generated fromphase matching between the soliton and resonant dispersive waves. Thisprocess occurs most efficiently when the soliton approaches the zerodispersion wavelength where the dispersion slope is negative. Pumpingmore energy into the fiber does not red-shift the soliton any further,but instead transfers the energy into the Cherenkov spectrum. As theinput pulse energy is increased from 1.45 nJ to 1.63 nJ (FIG. 2( a)),the soliton is still locked at a center wavelength of ˜1200 nm but moreenergy appears in the Cherenkov spectrum. Simulations suggest that anultrashort pulse can be filtered and compressed from this radiation toachieve energetic pulses across the zero-dispersion wavelength.

Both the wavelength shift and pulse energy can be significantlyincreased beyond what has been demonstrated through engineering of thefiber module. For example, simple dimensional scaling of the indexprofile can be used to shift the dispersion curve of the LP₀₂ mode.Numerical modeling shows that an output soliton energy of approximately2 nJ can be realized if the dispersion curve is shifted ˜100 nm to thelonger wavelength side. Additionally, pulse energy can be scaled byincreasing D·A_(eff). Aside from increasing the magnitude of dispersionthrough manipulation of the index profile and dimensions of the fiber,the effective area can be significantly enhanced by coupling into evenhigher-order modes. An effective area of ˜2000 μm² (more than 40 timesthis HOM fiber) was recently achieved by coupling to the LP₀₇ mode,which is hereby incorporated by reference in its entirety).

In summary, SSFS between 1064 nm and 1200 nm has been demonstrated in ahigher-order-mode, solid silica-based fiber. 49 fs Raman-shiftedsolitons were obtainable at 0.8 nJ with up to 57% power conversionefficiency. Due to the dispersion characteristics of the HOM fiber,Cherenkov radiation was also observed for appropriately energetic inputpulses. It is believed that HOM fiber should provide an ideal platformfor achieving soliton energies from 1 to 10 nJ for SSFS at wavelengthsbelow 1300 nm, filling the pulse energy gap between index-guided PCFsand air-core PBGFs. This intermediate pulse energy regime which couldnot be reached previously for SSFS could prove instrumental in therealization of tunable, compact, all-fiber femtosecond sources for awide range of practical applications.

In this work of the present invention, Cherenkov radiation has beengenerated at 1350 nm in an HOM fiber with 20% power conversionefficiency (approximately 25% photon efficiency). The Cherenkov outputpulses have been successfully filtered and compressed to 106 fs.Cherenkov radiation generated in the normal dispersion regime of thisHOM fiber can be used to extend frequency shifting even further, or tocreate a three-color femtosecond source (centered at the pump, frequencyshifted soliton, and Cherenkov radiation wavelengths). This new class offiber shows great promise for generating femtosecond pulses at variouswavelengths in the energy regime of several nJs.

The experimental setup is shown in FIG. 4( a). The pump source 20consists of a pulsed fiber laser (Fianium FP 1060-1S) centered at 1064nm, with 80 MHz repetition rate and 200 fs pulsewidth. We couple thesource into the HOM fiber module 22, which consists of a 12.5 cmstandard single mode fiber (flexcore) pigtail 24, 2.5 cm of long periodgrating (LPG) 26 and 1 m of HOM fiber 28. The combination of 12.5 cm ofSMF 24 and an LPG 26 of length 2.5 cm will provide an input signal tothe HOM fiber 28 that exhibits a linear chirp of at least 5.25 fs/nm.The LPG 26 converts the fundamental mode to the higher-order LP₀₂ modewith good (>90%) efficiency over a large (50 nm) bandwidth; for theinput wavelength of 1064 nm, 99% of the fundamental mode is converted tothe LP₀₂ mode, which exhibits anomalous dispersion in the HOM fiber 28between 908 and 1247 nm, see FIG. 4( b). At the input wavelength, theLP₀₂ mode, shown in FIG. 4( c), has an effective area A_(eff)=44 μm².The output of the HOM fiber module 22 is collimated and measured with anoptical spectrum analyzer 30 and a second order interferometricautocorrelator 32. A 1300 nm long-pass filter 34 is used to select outthe Cherenkov radiation, and a pair of silicon prisms 36, 38 are usedfor dispersion compensation and to simultaneously filter out anyresidual pump wavelength. A polarizer and a half-wave plate serve as avariable optical attenuator (VOA) 40 at the input of the HOM fibermodule 22.

We are able to couple a total power of 265 mW (3.31 nJ pulse energy)into the HOM fiber module 22. At this power level, the residual input,shifted soliton, and Cherenkov radiation can be clearly seen in theoutput spectrum shown in FIG. 5( a). The optical power residing in theCherenkov band is ˜53 mW (0.66 nJ pulse energy), a power conversionefficiency of 20% (25% photon conversion efficiency). We qualitativelymatch the experimental spectrum in simulation, shown in FIG. 5( b). Wenote the excellent qualitative match between simulation and experimentand the relatively good quantitative match. The observed quantitativediscrepancy could arise from our approximation of both the input sourcecharacteristics and the dispersion curve, which is not characterizedbeyond 1400 nm. This simulated spectrum corresponds to an input power of189 mW (2.36 nJ pulse energy), with 30% conversion to the Cherenkovband, equivalently 0.70 nJ in the Cherenkov pulse. At this power level,the soliton (centered at approximately 1200 nm) has shifted enoughenergy past the zero-dispersion wavelength so that resonant couplingoccurs efficiently at 1350 nm (Cherenkov radiation). Intuitively, growthof the Cherenkov radiation begins exponentially with increasing inputpower until the “spectral recoil” exerted by the Cherenkov radiation onthe soliton cancels the Raman self-frequency shift. After the soliton isfrequency-locked, for our experiment at 1200 nm, increasing the pumppower will only transfer energy to the Cherenkov spectrum instead ofshifting the soliton further. Simulation shows that up to approximately5 nJ can be pumped into the Cherenkov band, after which nonlineareffects begin to degrade the system. Experimental pulse energies werelimited by the pump source's non-Gaussian beam shape, which results inpoor coupling into the HOM fiber module.

We additionally measure Cherenkov output pulse energy as a function ofinput pulse energy by varying the attenuation (via VOA 40) at the inputof the HUM fiber module 22. We can see from FIG. 6 that the Cherenkovpulse energy increases rapidly at input energies of approximately 2 nJ(input power 160 mW). This “threshold” behavior, as well as the locationof the knee agrees well with our simulation. The threshold behavior hasalso been experimentally observed previously in PCF. A discrepancy inCherenkov pulse energy between numerical results and experiment wasfound at the highest input pulse energies we investigated, wheresimulation shows a faster increase in Cherenkov energy than theexperimental results. We currently do not have an explanation for thisdiscrepancy.

A second order autocorrelation trace of the filtered Cherenkov pulse atthe output of the HOM fiber module 22 is shown in FIG. 7( a); it isvisibly chirped and has an autocorrelation FWHM of 907 fs. We are ableto compress this pulse to 207 fs autocorrelation FWHM, shown in FIG. 7(b), with appropriate dispersion compensation by a silicon prism pair 36,38. We calculate the dispersion provided by the silicon prism pair(prism separation distance approximately 7 cm in optical pathlength) tobe β₂=−0.0065 ps² and β₃=−I.9×10⁻⁵ ps³. Applying such dispersioncompensation values to our spectrally matched simulation, we obtainnumerically an autocorrelation FWHM of 200 fs and a pulsewidth of 103fs. If we assume the same pulse shape, the experimentally measureddeconvolved pulsewidths with and without dispersion compensation are 106fs and 465 fs, respectively.

The location of the Cherenkov radiation can be tuned through engineeringof the fiber dispersion. For example, simple dimensional scaling of theindex profile of the HOM fiber 28 can be used to shift the dispersioncurve of the LP₀₂ mode. By shifting the zero-dispersion wavelength 50 nmto the shorter wavelength side, the generated Cherenkov radiation willalso shift by approximately the same amount. Such design control couldlead to the generation of useful femtosecond pulsed sources in spectralregimes unattainable by current laser systems. Furthermore, the largeeffective area and flexibility for dispersion engineering in the HOMfiber open up the possibility to achieve pulse energies significantlybeyond the level demonstrated here.

Although not demonstrated in our experiment, the generated Cherenkovpulse can be converted back to the fundamental mode by another LPG atthe output of the HOM fiber module. With proper dispersion matching,efficient and broadband (>100 nm) LPG has already been experimentallydemonstrated for mode conversion. Such an LPG will ensure the outputpulse is always converted back to a Gaussian profile, within the tuningrange. An important consideration for the output LPG is its length.Since the energetic output pulses are solitons for a specificcombination of dispersion and A_(eff) of the LP₀₂ mode, nonlineardistortions may occur when the energetic pulse goes to the (smallerA_(eff)) fundamental LP₀₁ mode at the output. However, the length overwhich the signal travels in the LP₀₁ mode, and hence the distortion itaccumulates, can be minimized because the high-index core of the HOMfibers enable LPG lengths of <5 mm. This implies that light can residein the LP₀₁ mode for <2.5 mm, hence largely avoiding nonlineardistortions. Note that the requirement for short LPGs actuallycomplements the need for broad-bandwidth operation, since the conversionbandwidth is typically inversely proportional to the grating length. Onthe other hand, depending on the intended usage, the higher-order-modeoutput could also be used directly without mode conversion.

In summary, we demonstrate a method of generating short pulses at 1350nm by exciting Cherenkov radiation in a HOM fiber with a 1064 nm pulsedfiber source. We have successfully dechirped a 465 fs pulse at theoutput of the HOM fiber to a 106 fs pulse with a pair of silicon prisms.This method of generating short pulses at 1350 nm can potentially beextended to other wavelengths and to higher pulse energies withappropriate design of the HOM fiber.

Example 2 All Fiber, Wavelength Tunable Femtosecond Sources forBiomedical Spectroscopy and Imaging

To emphasize the significance of the proposed femtosecond sources, wecompare our proposed sources with the existing mode-locked Ti:S laser,Ti:S pumped OPO and femtosecond fiber sources. FIG. 8 shows thewavelength tuning range of the sources. The absorption spectrum of wateris also shown to indicate the relevant wavelength range for biomedicalimaging. The arrows indicate the tuning ranges of a femtosecond Ti:Slaser, a Ti:S laser pumped OPO, and the proposed sources. The solidcircles represent the wavelength of existing femtosecond fiber lasers.The tuning range that has already been demonstrated in a preliminarystudy is also indicated. In essence, we want to develop two all-fiberfemtosecond sources that cover approximately the same wavelength windowas the existing Ti:S laser and Ti:S pumped OPO. These wide wavelengthtuning ranges were simply impossible to achieve in any existing fibersources, but are crucial to satisfy the requirements of nonlinearbiomedical engineering.

TABLE 1 Comparisons of femtosecond laser systems pulse energy (nJ)*pulse wavelength size free fiber width tuning tuning (cubic estimated fslasers space delivered (fs) range (nm) speed (s) feet) cost** $k Ti:S 255 ~60  700-1000 >10 ~10 170 Ti:S pumped 4 1 ~100 1120-1340 >10 ~14 250OPO Cr.Forsterite 3 1 ~60 1230-1280 >10 ~4 68 Current 15 5 ~2001030-1070 >10 ~1 40 1030 fiber source Current 15 5 ~200 1540-1590 >10 ~155 1550 fiber source proposed 10 10 ~50  775-1000 ultrafast ~1.5 70source at 775 to 1000 proposed 10 10 ~50 1030-1280 ultrafast ~1.5 50source at 1030 to 1280 *The pulse energies listed are all at the peak ofthe wavelength tuning range. **The estimated cost for the existing lasersystems are based on written price quotes from commercial vendors, Theestimated cost for the proposed sources are largely based on the priceof existing sources at 1550 and 1030 nm, with our best effort estimatesfor the additional cost of the HOM module and the control electronics.We have also included necessary cost for frequency doubling for thesource at 775 nm.

Table 1 compares some of the key characteristics of the existing and ourproposed femtosecond sources. The proposed systems would be much lessexpensive than the currently used state-of-the-art single box Ti:Slasers (Spectra-Physics Mai Tai and the Coherent Chameleon), probably ⅓to ¼ the cost. The telecom manufacturing platform employed in theproposed fiber sources provides an inherent opportunity for further costreduction by volume scaling. In addition, there are the practicaladvantages offered by the all-fiber configuration, such as a compactfoot print and a robust operation. However, what truly sets the proposedfemtosecond sources apart from other existing fiber sources isperformance. Table 1 shows that the proposed all-fiber sources willachieve comparable or better performances in terms of output pulseenergy, pulse width, and wavelength tuning range when compared to bulksolid-state mode-locked lasers. We note that the output characteristicsof the proposed sources listed above are delivered through an opticalfiber. The eliminate of the free-space optics makes the proposed fibersources more efficient in delivering power to an imaging setup. Thus,even at a slightly lower output power, the imaging capability of theproposed sources will likely be close to that of the free-space Ti:Slaser. It is worth emphasizing that significant research and developmentefforts have been devoted to femtosecond fiber sources in the last 15years or so. However, femtosecond fiber lasers have so far failed tohave a major impact in biomedical research. We believe the reason forthe low penetration of fiber femtosecond sources in the biomedical fieldis precisely due to various performance handicaps (such as pulse energy,wavelength tunability, pulse width, fiber delivery, etc.) that keptexisting fiber sources from being the “complete package.” It has nothingto do with the lack of demand or interest from biomedical researchers.Leveraging major developments, we believe we have finally arrived at thestage where all-fiber femtosecond sources can be realized withoutsacrificing performance. The successful completion of this researchprogram will make femtosecond sources truly widely accessible tobiologists and medical researchers and practitioner.

This program explores a new route for generating energetic femtosecondpulses that are continuously tunable across a wide wavelength range,where, in contrast to previous approaches, ultrafast pulses arewavelength shifted in a novel HOM fiber module by SSFS. By eliminatingthe constraint of a broad gain medium to cover the entire tuning range,our approach allows rapid, electronically controlled wavelength tuningof energetic pulses in an all-fiber configuration. FIG. 9 schematicallyshows the design of the proposed excitation sources. We start off with asingle wavelength femtosecond fiber source 50 at 1030 nm (or 775 nm withfrequency doubling from 1550 nm) with high pulse energy (10 to 25 nJ).The pulse is then propagated into a specifically designed HOM fibermodule 52 for wavelength shifting via SSFS. The output wavelength of thesoliton pulses are controlled by the input pulse energies (and/or HOMfiber length) using a pulse controller 54. The target performances ofthe proposed systems are 5- to 10-nJ pulses tunable from (1) 775 to 1000nm and (2) 1030 to 1280 nm in an all-fiber configuration.

A feature of the proposed research is to harvest the recent developmentin femtosecond fiber sources and the latest breakthrough in fiber opticcommunication industry. During the course of our research anddevelopment in both academia and industry over the last 5 years, we haveaccumulated significant amount of preliminary data to support ourapproach. Specifically, we present below our studies on femtosecondfiber sources, HOM fibers, and SSFS, three key components of theproposed femtosecond source.

The performance of fixed wavelength femtosecond fiber sources at 1030and 1550 nm have been improved significantly in the last several years.In fact, cost effective commercial fiber sources that are capable ofdelivering ˜10-nJ pulse energies at 40 MHz repetition rate or higheralready exist. These sources are mostly based on fiber chirped pulseamplification (CPA), where a low pulse energy oscillator serves as aseed source for the subsequent optical fiber amplifier. Examples of suchsources are offered by PolarOnyx Inc. and several other companies. FIG.10 shows the output spectrum, pulse width (autocorrelation), and thephotograph of the device (in FIGS. 10( a), (b) and (c), respectively).The output pulse energy of the source is 14.9 nJ, and the repetitionrate is 42 MHz. These sources will be sufficient to achieve our firstgoals of 1- to 2-nJ output pulse after SSFS.

One of the drawbacks of the commercial fiber sources is that they employthe CPA technique to achieve the pulse energies required for ourapplication. The combination of oscillator and amplifier inevitablyincreases the cost of the system. Obviously, a lower cost approach willbe to build a fiber oscillator that can achieve the pulse energydirectly. A series of advances in femtosecond fiber lasers atwavelengths around 1 μm, based on ytterbium-doped fiber (Yb:fiber) havebeen reported. These include some of the best performances reported forfemtosecond fiber lasers, such as the highest pulse energy (14 nJ),highest peak power (100 kW), highest average power (300 mW) and highestefficiency (45%). These are the first fiber lasers with pulse energy andpeak power comparable to those of solid-state lasers. These lasers arediode-pumped through fiber spliced to the gain fiber, and are thereforealready stable and reliable laboratory instruments. Uninterruptedoperation for weeks at a time is routine, except when the performance ispushed to the extremes of pulse energy or pulse duration.

The science that underlies the increases in pulse energy and peak powerlisted above is the demonstration of pulse propagation withinwave-breaking. The theoretical and experimental demonstration of“self-similar” evolution of short pulses in a laser is a majorbreakthrough. This is a completely new way to operate a mode-lockedlaser. The laser supports frequency-swept (“chirped”) pulses that avoidwave-breaking despite having much higher energies than prior fiberlasers. The pulses can be dechirped to their Fourier-transform limit(FIG. 11, far-right panel), but the chirped output is actuallyadvantageous to the design of the proposed tunable source. Asillustrated by FIG. 11, the experimental performance of a self-similarlaser agrees with the theoretical spectral and temporal pulse shapes.This will allow us to use the theory to scale the pulse energy to whatis needed for the present project, as well as to design self-similarlasers at 1.55 μm based on erbium-doped fiber (Er:fiber). The maximumpulse energy reported from a femtosecond Er:fiber laser remains at ˜1nJ, because there has been no attempt to develop self-similar lasers at1.55 μm yet.

The high-energy lasers described above are experimental systems. Theyemploy some bulk optical components in the cavity, such as diffractiongratings for anomalous group-velocity dispersion. These componentsnaturally detract from the benefits of the fiber medium, and integratedversions of these devices will be needed for most applications.Virtually all of the components of the lasers are now available in fiberformat, and several advances toward the ultimate goal of all-fiber andenvironmentally-stable devices were made in the past few years. Thefirst step is to replace the diffraction gratings with a fiber device.Microstructure fibers, which have become commercially available in thepast couple years, offer new combinations of dispersion andnonlinearity. The demonstration of dispersion control with a PCF was thefirst such application of microstructure fibers. The resulting laser islimited to low pulse energies by the small A_(eff) of the PCF. Theextension of this approach to air-core PBF is quite promising as it willenable all-fiber lasers capable of wave-breaking-free operation.

Lasers with segments of ordinary fiber are susceptible to environmentalperturbations such as strain or temperature changes. For ultimatestability, it will be desirable to construct lasers withpolarization-maintaining fiber. We exploited the fact thatphotonic-bandgap fiber is effectively a polarization-maintaining fiberowing to the high index contrast, to build the firstenvironmentally-stable laser at 1 μm wavelength. This laser operatesstably when the fiber is moved, twisted or heated. All the components ofthe laser (which was a testbed for new concepts) now exist in fiberformat. It is therefore now possible to design lasers in which the lightnever leaves the fiber, and which are impervious to environmentalperturbations.

Our development in robust femtosecond fiber lasers has already attractedsignificant commercial interests. PolarOnyx, Inc. (Sunnyvale, Calif.),and Clark/MXR, Inc. (Dexter, Mich.) have introduced products based onthe lasers described above (see FIG. 10 for the PolarOnyx source). Theappearance of commercial products two years after the initial reports ofnew concepts is evidence of the robust nature of the pulse-shaping inthe lasers.

The present invention is directed to an arrangement for producing highenergy, femtosecond output light pulses over a tunable wavelength rangefor wavelengths less than 1300 nm, using a relatively new type offiber—higher-order-mode (HOM) fiber—that yields strong anomalousdispersion in the output wavelength range. Advantageously, the HOM fiberis an all-solid silica fiber structure (i.e., does not include air gapsor other microstructures) where the guidance mechanism is conventionalindex guiding. This represents a major breakthrough in fiber design,inasmuch as it was not previously considered possible to obtainanomalous dispersion at wavelengths shorter than 1300 nm in anall-silica optical fiber.

In accordance with the present invention, a higher-order-mode (HOM)fiber has been developed that is capable of achieving a strong positive(anomalous) waveguide dispersion (D_(w)) for the LP₀₂ mode atwavelengths less than 1300 nm. In particular, an HOM fiber has beenformed that exhibits +60 ps/km-nm dispersion for the LP₀₂ mode in the1060-nm wavelength range. Combined with in-fiber gratings, this resulthas enabled the construction of an HOM anomalous dispersion element(hereinafter referred to as an “HOM module”) with low loss (˜1%), and aneffective area A_(eff) (e.g., ˜44 μm²) that is ten times larger thanconventional photonic crystal fibers (PCFs). Significantly, the guidancemechanism is index-guiding, as in standard fibers. Therefore, theinventive HOM fiber retains the desirable properties of such fibers,including low loss, bend resistance, and lengthwise invariance (in termsof loss, dispersion, etc.), making such a fiber attractive for a varietyof applications. By utilizing the phenomenon of SSFS, for example, aninput optical signal at a first, input wavelength can be shifted to asecond, output wavelength after propagating through the HOM fiber of thepresent invention. Additionally, an HOM fiber module in accordance withthe present invention can be used as a femtosecond fiber source at 1300nm using soliton Cherenkov radiation in the HOM fiber to efficientlyconverter a 1030 nm femtosecond fiber source to the desired 1300 nmwavelength.

FIG. 12 provides an intuitive picture for the dispersive behavior of theguided modes by comparing the properties of the LP₀₁ mode typicallyassociated with convention single mode fiber, and the LP₀₂ mode assupported within the inventive HOM fiber. In particular, FIG. 12( a)shows modal images for the fundamental LP₀₁ mode (top) and the higherorder LP₀₂ mode (bottom). FIG. 12( b) shows the evolution of these modeprofiles as a function of wavelength, in particular at 800 nm, 1040 nmand 1250 nm. The gray background in FIG. 12( b) is used to illustratethe refractive index profile of the fiber. As shown in the top set ofmodal images, the LP₀₁ mode monotonically transitions from the highindex central core to the surrounding lower index regions as thewavelength increases from 800 nm to 1040 nm, and finally to 1250 nm.Thus, the fraction of power traveling in lower index regions increaseswith increasing wavelength. Since the velocity of light increases as therefractive index of the medium drops, the LP₀₁ mode experiences smallergroup delays as wavelength increases.

Waveguide dispersion (D_(w)), which is the derivative of group delaywith respect to wavelength, is thus negative for the LP₀₁ mode.Therefore, in wavelength ranges in which material dispersion (D_(m)) isitself negative, the conventional LP₀₁ mode can achieve only negativetotal dispersion values, where “total dispersion” D_(total) is definedas the sum of waveguide dispersion and material dispersion. This isillustrated in FIG. 12( c) (top), which plots material dispersion D_(m)as well as total dispersion D_(total) of the LP₀₁ mode in the 1060-nmwavelength range.

In contrast and in accordance with the present invention, thehigher-order LP₀₂ mode is designed to have the mode evolution shown inFIG. 12( b) (bottom). Again, the gray background is used to illustratethe refractive index profile for the fiber supporting this mode. Asshown, when the wavelength increases from 800 nm to 1040 nm, and then to1250 nm, the mode evolves in the opposite direction as the conventionalfiber described above. That is, with reference to the diagrams along thebottom of FIG. 12( b), the mode transitions from the lower index regionsto the higher index core as the wavelength increases from 800 nm to 1250nm. The LP₀₂ mode thus experiences larger group delays as the wavelengthincreases.

Therefore, the LP₀₂ mode will exhibit a wavelength dispersion D_(w) thatis positive over this entire range as the mode transitions from thecladding to the core. This is illustrated in FIG. 12( c) (bottom), whichshows the wavelength range where this transition occurs. Indeed, verylarge positive values of D_(w) may be obtained, vastly exceeding themagnitude of the material dispersion D_(m) (which, as mentioned above,is negative over the same range). As a result of the substantialdifference in magnitude between the waveguide dispersion and thematerial dispersion, the LP₀₂ mode that propagates along an HOM fiberwill exhibit a total dispersion D_(total) that is positive (anomalousdispersion).

It is to be noted that this evolution is governed by the “attractive”potential of various high index regions of the waveguide, and can thusbe modified to achieve a variety of dispersion magnitudes, slopes andbandwidths. This yields a generalized recipe to obtain positivedispersion in a variety of wavelength ranges.

FIG. 13 shows the index profile of an exemplary HOM fiber formed inaccordance with the present invention to provide this positivedispersion value, where a broad, low index ring 100 serves tosubstantially guide the LP₀₂ mode at shorter wavelengths. As describedabove, the mode will then transition to a small, high index core 120 aswavelength increases (as described above in association with FIG. 12(b), bottom). The experimentally recorded near-field image of this LP₀₂mode is shown in FIG. 14, where measurements have shown that thisexemplary HOM fiber will exhibit an effective area A_(eff) ofapproximately 44 μm² at 1080 nm.

The well-known physics of SSFS dictates that the wavelength tuning rangeis limited by the range within which the dispersion of the fiber mode isanomalous (positive). In other words, for a tuning range of λ_(tuning),the dispersion-zero crossings of the dispersion curves must also beseparated by at least the same amount λ_(tuning). For many applications,it is desirable that this range be at least 300 nm. More broadly, atuning range anywhere between 10 nm and 2000 nm may be considereduseful. In general, the range of such tuning, and correspondingly theenergy carried by the shifted soliton, scale with D*A_(eff) for thewavelength and the mode in which the soliton signal resides.

The well-known physics of generation of Cherenkov radiation, on theother hand, requires the existence of a zero-dispersion crossing. If anoptical soliton exists in its vicinity in the anomalous dispersionwavelength range, then Cherenkov radiation is generated in the spectralregion on the other side of this zero-dispersion wavelength—i.e., in theregion where the dispersion is normal. The exact spectral location ofthe generated wave is further governed by the dispersion slope of thefiber mode. Again, the energy of the converted radiation scales asD*A_(eff) for the mode in which the optical radiation resides.

In accordance with the present invention, therefore, the fiber designproblem reduces to one of configuring an HOM fiber with the requiredvalue of D*A_(eff) at the output wavelengths of the dispersion curve.The general fiber index profile for achieving D_(w)>0 for the LP₀₂ modeis shown in FIG. 15. While FIG. 12 provides the physical intuition forD_(w)>0 in an HOM fiber, achieving target dispersion D and effectivearea A_(eff) values requires a numerical optimization of the sixparameters shown in FIG. 13, namely, the indices and dimensions of ring100, trench 140 and core 120. There are two ways to achieve a largedispersion (D) value; one is by increasing refractive index valuesΔN_(core) and ΔN_(ring), but this may be at the expense of the effectivearea A_(eff). The second approach is by increasing r_(ring) as well asr_(trench). Increasing r_(ring) will enhance the mode size, whileincreasing r_(trench) will provide for greater effective index changesas the mode transitions, resulting in larger dispersion.

The key to achieving the desired properties is a mode that cantransition (as a function of wavelength) through well-defined, sharpindex steps in the fiber's index profile. Therefore, the fabricationprocess must be capable of producing both large index steps as well assteep index gradients, as shown in FIG. 15. The ideal means to achievethis is the Modified Chemical Vapor Deposition (MCVD) process, whichaffords the best layer-to-layer control of refractive index of allestablished fabrication technologies for fibers.

Dimensional scaling of the preform can also be used to shift thewaveguide dispersion D_(w) in order to achieve the D*A_(eff) necessaryfor the desired output wavelength ranges. This is known in the art ofoptical waveguides as complementary scaling, which states thatwavelength and dimension play a complementary role in the wave equationand, therefore, are interchangeable. However, it is to be noted thatthis is true only for the waveguide component of dispersion, D_(w).Changes in the material dispersion, D_(m), are not complementary and, asa result, the total dispersion D is not wavelength scalable. In otherwords, to move the dispersion curve that provides satisfactory operationin the 1030 nm wavelength range to the 775 nm spectral range, thedispersion D_(w) needs to be high enough to counteract the strongnegative trend for D_(m) as wavelength decreases. Therefore, achievingsimilar properties at lower wavelengths needs the use of bothdimensional scaling and the above-described dispersion-increasingconfigurations.

To achieve the higher 5- to 10-nJ output pulse energies, the design ofan inventive HOM in this range requires a D*A_(eff) value that is fiveto ten times greater than that associated with providing output pulsesin the 1-2 nJ range. The main difficulty is to simultaneously achievethe large values of D*A_(eff) while maintaining λ_(tuning) atapproximately 300 nm. FIG. 16 illustrates the simulated total D vs.wavelength curves for a variety of acceptable profiles, where thematerial dispersion value of silica, D_(m), is also shown. An importantconstraint applied in generating the profiles shown in FIG. 16 is thatthe effective index n_(eff) of the HOM in which signal propagation isdesired (i.e., the mode for which the dispersion curves are shown), isvastly separated from the n_(eff) of any other mode that may be guidedin the fiber. The large separation in n_(eff) between modes ensures thatthe signal that is introduced in the HOM predominantly propagates onlyin that mode and does not randomly coupled to any other mode. Suchrandom coupling may occur due to bends and other environmentalperturbations, and typically the n_(eff) difference between the modesshould be greater than 10⁻⁵ to avoid this type of coupling.

FIG. 17 shows an example of the designed and fabricated index profilesfor an HOM fiber formed in accordance with the present invention thatyields a large positive dispersion in the 1060-nm wavelength range. Thepreform profiles closely match the design profile in both index valuesand the steep index gradients. Also shown in FIG. 17 are index profilesfrom different sections of the preform, showing the uniformity of theMCVD process in fabrication an HOM fiber whose properties are invariantas a function of fiber length. This robust fiber fabrication process iscritical to provide a constant zero-dispersion wavelength in an HOMfiber for SSFS, and is a significant advantage of the inventive HOMfiber over the prior art bandgap fibers.

FIG. 18 illustrates an exemplary wavelength converting HOM module 60formed in accordance with the present invention, including a section ofHOM fiber 62 having the characteristics as described above inassociation with the above FIGS. 15-17 to provide high energyfemtosecond pulses at wavelengths less than 1300 nm. In accordance withthe present invention, HOM module 60 utilizes SSFS, or a combination ofSSFS with Cherenkov radiation, to shift the wavelength of an incomingsignal to an output wavelength selected for a specific application (theoutput wavelength less than 1300 nm).

Further, in accordance with the present invention, HOM module 60provides for dispersion compensation prior to wavelength shifting, suchthat chirped incoming pulses (i.e., pulses with a linear chirp of atleast 5.25 ps/nm) are “de-chirped” with the required amount ofdispersion within HOM fiber 62. Thereafter, the de-chirped pulsesundergo SSFS and/or Cherenkov radiation to generate the output pulses atthe desired wavelength.

For proper operation of HOM module 60, an input mode converter 64 isneeded to convert an incoming Gaussian-shaped LP₀₁ mode signal into thedesired LP₀₂ mode. One preferred method for providing the modeconversion is with one or more in-fiber long period gratings (LPGs).This type of grating can be permanently formed in fibers bylithographically transferring a grating pattern from an amplitude maskto the fiber using a UV laser. For efficient grating formation, thefiber is typically saturated with deuterium, which acts as a catalystfor the process, resulting in UV-induced index changes in thegermanosilicate glass. In another embodiment, the input mode convertermay convert any arbitrary incoming spatial profile of light into the HOMthat is desired to be propagated in the HOM fiber. For someapplications, an output mode converter may be used to transform thehigher-order-mode into another spatial mode. In the illustration of FIG.18, an output mode converter 66 is shown as disposed at the output ofHOM fiber 22 to transition the wavelength-shifted LP₀₂ mode signal backinto a conventional LP₀₁ signal. More generally, an output modeconverter can be used to convert the HOM into any desired spatialprofile of light.

Alternatively, in some applications, it may be desired that no outputmode converter is used, inasmuch as the wavelength-shifted radiationalready exhibits the desired spatial mode profile. In these cases,therefore, the need for an output mode converted is obviated. In yetanother embodiment, the HOM module may comprise a plurality of separateHOM fiber sections coupled together in series, using fiber splicingtechniques or another mode converter to join together the adjacentsections. If they are joined by splices, the HOM in the first fiber isexpected to adiabatically transition to the same mode order in thesecond fiber. If they are joined together by means of a mode converter,on the other hand, the mode order from the first fiber to the secondfiber can also be changed. Such arrangements may be desired inapplications where, in order to increase the λ_(tuning) for SSFS, twoconcatenated sections will provide a much larger tuning range than thatassociated within only a single HOM fiber section. Alternatively, sucharrangements may allow for changing the dispersion slope of thezero-dispersion crossing, as may be required for adjusting thewavelength at which Cherenkov radiation occurs. In the case where a modeconverter is used to join two sections of HOM fiber, it is known fromthe prior art that such mode converters may be tunable, with thecapability of switching light from one incoming HOM fiber to any of aset of outgoing HOM fibers (including, of course, reflecting back intothe incoming HOM fiber). If a tunable mode converted is employed in thiscase, the resulting HOM module will additional provide a means todynamically change the effective optical path length of the fiber and,by extension, its dispersion, dispersion-zero and/or dispersion slope(as may be desired for different SSFS and Cherenkov applications). Thus,a module with adjustable HOM fiber lengths may be designed and isconsidered to fall within the scope of the present invention. Indeed, inembodiments that utilize a multiple number of concatenated HOM fibersections, tunable mode converters may be used at the interface betweenany two sections.

LPGs offer coupling between co-propagating modes of a fiber and havefound a variety of applications as spectral shaping elements andmode-conversion devices. However, LPGs are normally narrow-band devices,and while they offer strong mode coupling (>99%), the spectral width ofsuch coupling was typically limited to a range of 0.5 to 2 nm, toonarrow for a femtosecond pulse. To overcome the spectral limitation,reports have shown that the LPG bandwidth can be extended to greaterthan 60 nm in some cases, if the fiber waveguide is configured to yieldtwo modes with identical group velocities. It is to be noted that thelarge bandwidth of HOM module 20, as shown in FIG. 19( a) (i.e.,approximately 51 nm), is uniquely enabled by the dispersive design ofthe fiber, which enables matching the group velocities of the twocoupled modes. It is a significant aspect of the present invention thatthe utilization of the LPGs allows for the formation of an “all-fiber”tunable femtosecond pulse source.

Referring again to FIG. 18, HOM module 60 is spliced to an input singlemode fiber 70 at input long period grating 64. The splice between singlemode fiber 70 and input LPG 64 is configured such that the signalpredominantly resides in the LP₀₁ mode, thus ensuring mode conversionwith high efficiency and also minimizing signal propagating in any modeother than the desired mode. This enables a device to be constructed inwhich the signal experiences high modal stability, even in the presenceof bends and other environmental perturbations. Indeed, input singlemode fiber 70 may be the output fiber of a laser source (not shown),avoiding any spurious mode coupling, especially in systems where thechirped output of the laser source needs to be directly coupled into theHOM fiber module.

As mentioned above, output long period grating 66 is used to convert thebeam back to a Gaussian output. Dispersion-matching configurations arepreferably used that yield ultra-large bandwidths, ensuring that theoutput pulse is always converted back to a Gaussian profile, within atuning range of approximately 250 nm. An important consideration foroutput grating 66 is its length. Since the energetic output pulses aresolitons for the specific combination of dispersion D and effective areaA_(eff) of the LP₀₂ mode, nonlinear distortions may occur when thesignal converts to the LP₀₁ mode (having a smaller A_(eff)) at theoutput. However, the length over which the signal travels in the LP₀₁mode, and hence the distortion it accumulates, can be minimized. Thehigh index core of HOM fiber 62 enables the use of an output long periodgrating 66 of lengths less than 5 mm, which implies that light residesin the LP₀₁ mode for less than 2.5 mm and therefore largely avoidsnonlinear distortions. It is to be noted that the requirement for“short” LPGs actually complements the need for broad bandwidthoperation, since the conversion bandwidth is typically inverselyproportional to the grating length.

FIG. 19( b) shows the central parameter of interest—the dispersion ofthe LP₀₂ mode, as measured by spectral interferometry. The dispersion is+60 ps/nm-km at 1080 nm. The A_(eff) of this fiber (44 μm²) is an orderof magnitude larger than PCFs with similar dispersion (PCF A_(eff)˜4μm²), and is in fact larger than that of commercial SMFs at thesewavelengths (SMF A_(eff)˜32 μm²). FIG. 19( c) again illustrates thedispersion of the LP₀₂ mode, in this case over the narrow range of 1040to 1120 nm. The dispersion is +60 ps/nm-km at 1080 nm. The A_(eff) ofthis fiber (44 μm²) is an order of magnitude larger than PCFs withsimilar dispersion (PCF A_(eff)˜4 μm²), and is in fact larger than thatof commercial SMFs at these wavelengths (SMF A_(eff)˜32 μm²).

There are a number of theoretical and experimental works on SSFS in thepast, including some targeting biomedical applications. Reports havedemonstrated SSFS in a number of fiber structures within the last 5years. Previously, a novel tapered air-silica microstructure fiber wasfabricated and demonstrated SSFS within the telecom window of 1.3 μm to1.65 μm in a 10-cm long tapered microstructure fiber (inset in FIG. 20(b)). By varying the input power into the fiber, cleanself-frequency-shifted solitons were observed with a maximum wavelengthshift of ˜300 nm (FIG. 20( a)). Over 60% photons were converted to thefrequency-shifted soliton. The experimental dependence of solitonwavelength shift upon the incident power is shown in FIG. 20( b).Similar experiments were also demonstrated using a mode-locked fiberlaser and PCF, shifting of the pulse wavelength continuously from 1 to1.3 μm, with ˜1 m of photonic-crystal fiber (FIG. 21). Despite theseearly works by ourselves and colleagues in the field, the highestsoliton pulse energy of 0.1 to 0.4 nJ were obtained at 1030 to 1330 nm,still substantially below 1 nJ.

Our recent breakthrough in the HOM fiber provides an exciting newopportunity for SSFS at the practical pulse energies of 1 to 10 nJ andat wavelength below 1300 nm. We have experimentally investigated thebehavior of SSFS at Cornell using the HOM fiber module provided by OFS.FIGS. 22 and 23 respectively show the experimental setup and results.Despite the fact that the HOM fiber module we used for the demonstrationwas designed for telecommunication purposes and was not ideally suitedfor SSFS at 1060-nm input, and the fact that the input pulse (inset inFIG. 23) from our commercial fiber source (Fianium, UK) is far fromperfect, our preliminary results unequivocally demonstrated thefeasibility and promise of the approach proposed.

The maximum power that we were able to launch from our sourcecorresponds to 2.0 nJ pulse into the HOM fiber. The output spectrum(FIG. 24( a)) was measured by a spectrometer. The Cherenkov radiationwas filtered out using a free space optical filter. After dispersioncompensation to remove the linear chirp in the Cherenkov pulse, secondorder interferometric autocorrelation was performed to measure the pulsewidth (inset in FIG. 24( a), horizontal scale is 2 ps total delay). Tocompare with the experimental data, we have also carried out numericalsimulations using the standard split-step Fourier transform method, andthe results are shown in FIG. 24( b).

Although it was our very first attempt, and the HOM fiber was not fullyoptimized for soliton Cherenkov radiation, our preliminary resultsunequivocally demonstrated the feasibility and promise of the approachproposed. FIG. 24 clearly shows that:

1. An output pulse width of 100 fs FWHM was obtained.

2. An output pulse energy of 0.046 nJ was obtained at the centerwavelength of ˜1.36 μm.

3. Remarkable agreement between experiments and numerical modeling wasachieved without any fitting parameter, demonstrating the reliability ofour numerical prediction.

We emphasize here that the low Cherenkov energy is entirely due to themaximum input pulse energy (2.0 nJ) with our fiber source at 1.06 μm. Infact, negligible Cherenkov radiation was observed even at input pulseenergy of 1.4 nJ. As we will show in the next section, our numericalmodeling indicated that the existing HOM fiber should be able to provideapproximately 2-nJ Cherenkov radiation at 4-nJ input. The key resultsare summarized below:

1. A continuous wavelength shift of ˜130 nm (1060 to 1190) was achieved.

2. An output pulse energy of 0.84 nJ was obtained at 1.39-nJ inputpulse.

3. A high quality output pulse with ˜50-fs FWHM and a high conversionefficiency (i.e., the amount of optical power that is transferred to thewavelength shifted soliton) of ˜60% were obtained despite of the lowquality input pulse.

4. Remarkable agreement between experiments and numerical modeling wereachieved despite of the non-ideal input, demonstrating the robustness ofsoliton pulse shaping.

We note that at the highest input pulse energy, a new spectral peakappeared at much longer wavelength (˜1350 nm). This is the well-knownresonance Cherenkov radiation of the soliton due to the negativedispersion slope, which is also predicted by our simulation (FIG. 24(b)). The onset of the Cherenkov radiation sets the long wavelength limitof the wavelength tuning range using SSFS and is highly predictable bythe zero dispersion wavelength of the fiber.

Our initial success of SSFS in a HOM fiber module, and our provencapability to numerically predict the behavior of SSFS in a HOM fibergive us a high degree of confidence to achieve the stated goals. Throughextensive numerical simulations, we have already determined the requireddispersion (FIG. 25) and A_(eff) of the HOM fibers to achieve our firstgoal of 1- to 2-nJ pulses, tunable from 775 to 1000 nm and 1030 to 1280nm.

Additionally, our initial success in also generating Cherenkov radiationin an HOM fiber module, and our proven capability to numerically predictthe behavior of Cherenkov radiation in an HOM fiber, also gives us ahigh degree of confidence to achieve the stated goals. Based on theresults of our preliminary experiments, we essentially need to performtwo tasks: (1) to convert the wavelength to 1300 nm instead of the 1360nm demonstrated; (2) to increase the output pulse energy to greater than10 nJ instead of the fraction of a nJ demonstrated. The centerwavelength of the Cherenkov radiation is mostly determined by the zerodispersion wavelength and the TOD; while the Cherenkov energy isdetermined by the magnitude of D*A_(eff). Thus, the first task can beaccomplished by wavelength-shifting the dispersion curve, and the secondby increasing the dispersion and/or effective area of the HOM fiber.

A remarkable feature of soliton Cherenkov radiation is its robustnessagainst input variations. For example, at 3.2 nJ input, our numericalmodeling predicts Cherenkov radiation at 66 fs pulse width and 2 nJ,results that are very similar to those obtained at 4.2 nJ input.Numerical simulations further showed that fiber length variation from 25to 45 cm essentially provided identical output characteristics exceptthat the values for dispersion compensation need to be adjustedaccordingly to achieve the shortest pulse. We note that the designeddispersion curve shown in FIG. 26( a) (dashed line) is of the samedispersion value as our existing HOM module except that the peakwavelength is shifted for optimum output at 1300 nm. Preliminary designsimulations by the inventor indicated that such dispersioncharacteristics are readily achievable. We emphasize that these designstudies are based on reliable in-house design tools developed at OFS,and have taken into account practical considerations such as themanufacturability and yield of the fiber. Thus, these designs are viablecommercially.

FIG. 26( b) shows numerical simulation results of the Cherenkovradiation in such HOM fibers at 4.2 nJ input. The conversion efficiencyis ˜50% for a Gaussian input pulse at 280-fs width (FWHM). The outputpulse has a width (FWHM) of approximately ˜66 fs after dispersioncompensation to remove the linear chirp of the pulse. The pulse hasexcellent quality with more than 90% of the energy residing within thetime window that is twice the FWHM. We clarify here that the localminimum for water absorption is actually located at 1270 nm instead of1300 nm, even though it is commonly referred to as the 1300-nm spectralwindow. Thus, our output spectral filter (shown in FIG. 26( b) bottom)is centered at 1270 nm.

To achieve greater than 10 nJ pulse energy, the magnitude of D*A_(eff)must be increased by ˜5 times. FIG. 27( b) shows the numericalsimulation results of Cherenkov radiation with the dispersion curvesshown in FIG. 27( a). Greater than 13 nJ of Cherenkov radiation can beproduced in a HOM fiber as short as 3 cm. Both dispersion curves (solidand dashed lines in FIG. 27( a)) produced similar outputcharacteristics. Thus, our design is reasonably tolerant to smallamounts of deviation in the dispersion curve, which can be introduced inthe fiber fabrication process.

FIG. 28( a) shows numerical simulation results of SSFS in such HOMfibers, by adjusting the launch power into the HOM fiber module. Theconversion efficiency is ˜70% for a Gaussian input pulse at 280-fs width(FWHM). Thus, even a 5-nJ pulse launched into the HOM fiber moduleshould be sufficient to achieve the design specifications. The outputpulse widths are between 50 and 70 fs throughout the tuning range. Verysimilar results were also obtained for the 775-nm input with the designcurve shown in FIG. 25( a). We have further determined that a shift aslarge as ˜50 nm in zero-dispersion wavelength (the dash-dotted and thedotted line in FIG. 25( b)) will not significantly impact (<8% in outputpulse energy) the performance of the HOM fiber, making our designtolerant to fabrication imperfections. We note that the dispersioncurves shown in FIG. 25 are of the same functional dependence as ourexisting HOM module except that the peak wavelength is shifted foroptimum performance at 775-nm and 1030-nm input. Preliminary designsimulations indicated that such dispersion characteristics areachievable. In fact, dispersion characteristics better than those shownin FIG. 25 can be readily obtained. We emphasize that these preliminarydesign studies are based on highly reliable in-house design toolsdeveloped at OFS, and have taken into account practical considerationssuch as the manufacturability and yield of the fiber. Thus, thesedesigns are immediately viable commercially.

In addition to the power tuning of the output wavelength, an alternativemethod for wavelength tuning is simply using different fiber length.FIG. 28( b) shows the simulated output spectrum at various HOM fiberlengths while maintaining the input power. Tuning range identical tousing power adjustment, with a conversion efficiency of ˜70% can beeasily achieved.

Perhaps the most promising and successful area in biomedical imagingthat showcases the unique advantage of multiphoton excitation is imagingdeep into scattering tissues. One of the promising approaches forimaging deep into scattering biological tissue is using longerexcitation wavelength. It is well known that the scattering mean freepath is proportional to the fourth power of the excitation wavelength inthe Rayleigh region, where the size of the scatterer (a) is much smallerthan the wavelength, i.e., 2π/λ<0.1. When the size of the scattererbecomes comparable to the wavelength, i.e., in the Mie scatteringregion, the scattering mean free path (MFP) has a weaker dependence onthe wavelength. Although there is little data for tissue scatteringbeyond 1.1 μm, the available data at shorter wavelengths clearlyindicates the general trend that the scattering MFP increases as oneuses longer excitation wavelength. In fact, the “diagnostic andtherapeutic window,” which is in between the absorption regions of theintrinsic molecules and water, extends all the way to ˜1280 nm (see FIG.8 for the water absorption spectrum), significantly beyond the currentinvestigations of the near IR spectral window of 700 to 1000 nm. Webelieve such a constrained is mostly caused by the lack of a convenientexcitation source.

There are a few experimental demonstrations of imaging at longerwavelengths by several groups. We have also carried out detailed studiesof multiphoton excitation of fluorophores within the spectral windows of1150 to 1300 nm, and have found useful multiphoton cross sections (10 to100 GM, comparable to fluorescein at shorter wavelength) exist for anumber of long wavelength dyes (FIG. 29). Clearly, longer wavelengthimaging is feasible. In addition for the reduction of scattering of theexcitation light, there are a number of additional advantages at thelonger excitation window. It was shown previously that longer wavelengthimaging is less damaging to living tissues. The use of longer excitationwavelengths will typically result in longer wavelength fluorescenceemissions and second or third harmonic generations. Because of thescattering and absorption properties of tissues, a long wavelengthphoton stands a much better chance of being detected by the detector.Thus, the long wavelength window for multiphoton imaging should alsoimprove the signal collection, another critical issue in imagingscattering samples. There is no doubt that the creation of an all-fiber,wavelength tunable, energetic femtosecond source at the longerwavelength window of 1030 to 1280 nm will open significant newopportunities for biomedical imaging.

Our overall approach to wavelength-tunable sources is to develop fibersources of 10- to 25-nJ and ˜300-fs pulses, which will propagate in HOMfiber modules as Raman solitons to produce the desired outputs. Startingwith pulses at 775 nm (1030 nm), pulses tunable from 775 to 1000 (1030to 1280 nm) will be generated. The source development that we propose isenabled by the coincident advances in short-pulse fiber lasers andpropagation of higher-order modes, along with the commercial developmentof semiconductor structures for stabilizing short-pulse lasers (to bedescribed below). The availability of excellent fibers and continuedimprovement in the performance and cost of high-power laser diodesprovide the technical infrastructure needed to support the developmentof short-pulse fiber devices.

We will develop single wavelength all-fiber femtosecond sources at 1030nm and 775 nm with pulse energies at 10 and 25 nJ at repetition rates of40 to 100 MHz.

Our first step is to modify and optimize commercially availablefemtosecond fiber sources to achieve ˜10-nJ pulses, which will besufficient to achieve our first goal of 1- to 2-nJ output pulses.Although we are fully capable of building such sources ourselves, we aimto jump start the program by fully leveraging existing commercialtechnologies. The main task during this stage is to make the commercialsources truly all-fiber. We realized that one of the main drawbacks ofexisting commercial fiber sources is that they are not all-fiber. Forexample, the PolarOnyx system (FIG. 10( c)) requires a separate ratingcompressor box (not shown in the photograph) to de-chirp the outputpulse at 14-nJ output. As we have discussed, free-space components suchas the grating compressor not only negate many advantages of the fibersource, they also make the fiber source ironically incompatible withfiber delivery.

Energetic femtosecond fiber sources (either from an oscillator or a CPAsystem) have typically chirped output to avoid optical nonlinearity, andtherefore, external dispersion compensation is required to recover thefemtosecond pulses. The main reason for the required free-space gratingcompressor in the current fiber source is the lack of low nonlinearityanomalous dispersion fiber, i.e., fibers with large A_(eff) and largepositive D value. Although airguided BGF can be used for dispersioncompensation, there were a number of practical issues such astermination, fusion splice, birefringence, loss, etc. On the other hand,the proposed HOM fiber can easily perform dispersion compensation inaddition to SSFS, by simply adding HOM fiber length in the HOM fibermodule. For example, with a typical chirp of 0.24 ps²/nm from a fibersource (the amount of chirp caused by ˜12 m of SMF at 1030 nm), oursimulation shows that a HUM fiber length of ˜6 m will produce the outputnearly identical to that shown in FIG. 28. FIG. 30 shows the pulseevolution through 1 meter of standard SMF pigtail and approximately 6meter of HOM fiber starting with a typical output chirp of 0.24 ps²/nm.Intuitively, the first ˜3 meters of the HOM fiber, and the last meter orso of HOM fiber does the SSFS. Because the transmission loss of the HOMfiber is extremely low (similar to conventional SMF where light loseshalf of its power over a length of 10 miles), HOM fiber length of tensof meters will incur essentially zero loss. In fact, as we will explainlater in greater detail, the longer fiber length not only compensatespulse chirp from the fiber source, making the source all-fiber, it wouldsimultaneously offer a tremendous practical advantage in a clinicalenvironment.

FIG. 31 shows the pulse width and energy directly from the HOM fiber atthree propagation distances. Approximately 10 nJ pulses can be obtainedat 100 to 200 fs pulse width without any dispersion compensation if wereduce the HOM fiber length to 2.2 cm. Although the pulse quality andenergy is somewhat reduced (FIG. 31) when compared to that shown in FIG.27( b), the elimination of dispersion compensation is highly convenientand offers the opportunity for direct fiber delivery of such anenergetic pulse. We will pursue this exciting opportunity in addition toour conventional approach of post dispersion compensation.

The second step, which involves our own laser and source development,aims to improve the pulse energy to ˜25 nJ in an all fiber design. Suchpulse energies are necessary for achieving a final tunable output of 5-to 10-nJ pulses. There are two approaches to achieve our aim.

The first approach closely follows the strategy of the existingcommercial devices using CPA. In a realistic fiber amplifier capable ofthe needed performance, a pulse is taken from an oscillator by splicingon an output fiber (tens of meters in length) where the pulse is highlystretched temporally. The stretched pulse is then amplified to highpulse energy by a fiber amplifier. Nonlinear effects that could distortthe pulse are avoided because the pulse is stretched, which reduces thepeak power. The output from the amplifier will be an amplified versionof the same chirped pulse. The pulse is contained in ordinarysingle-mode fiber throughout the device. The above described CPA schemehas enabled significantly improved pulse energy in fiber amplifiers.Even μJ pulse energies can be obtained (although at much lowerrepetition rate). For our proposed sources, we will amplify pulses to 25nJ at 1030. We aim to amplify to 50 nJ at 1550 nm in order to obtain˜25-nJ pulses at 775 nm. Commercial fiber amplifier modules alreadyexist to delivery the necessary power for our applications. In addition,methods for overcoming fiber nonlinearity in a fiber CPA system havebeen demonstrated. Thus, we do not anticipate any difficulty inachieving these design goals.

The combination of a laser and an amplifier in our first approach allowsboth to be designed easily, and is certain to meet or exceed our designspecifications. Indeed, it is highly likely that commercial femtosecondfiber sources based on the CPA technique can deliver the necessary pulseenergy (25 to 50 nJ) and power (1 to 2 watts) within the grant period.Thus, there is a possibility that we can continue leveraging commercialfemtosecond fiber sources. On the other hand, the addition of anamplifier adds costs and complexity to the source (at least one morepump laser and driver will be required), and always adds noise to theoutput. Ultimately, it will be desirable to reach the needed pulseenergies directly from oscillators. Thus, as an alternative and lowercost approach, we will pursue the development of high-energy oscillatorsin parallel with the construction of low-energy oscillators that areamplified to the required energies.

The essential physical processes in a femtosecond laser are nonlinearphase accumulation, group-velocity dispersion, and amplitude modulationproduced by a saturable absorber. A real or effective saturable absorberpreferentially transmits higher power, so it promotes the formation of apulse from noise, and sharpens the pulse. Once the pulse reaches thepicosecond range, group-velocity dispersion and nonlinearity determinethe pulse shape. In the steady state, the saturable absorber thus playsa lesser role, stabilizing the pulse formed by dispersion andnonlinearity. It is known that the pulse energy is always limited byexcessive nonlinearity. This limitation is manifested in one or twoways:

(1) A high-energy pulse accumulates a nonlinear phase shift that causesthe pulse to break into two (or more) pulses. This is referred to as“wave-breaking.”

(2) To date, the best saturable absorber for fiber lasers is nonlinearpolarization evolution (NPE), which produces fast and strong amplitudemodulation based on polarization rotation. It was employed in the Ybfiber lasers described in our preliminary results. A disadvantage of NPEis that the transmittance is roughly a sinusoidal function of pulseenergy; the transmittance reaches a maximum and then decreases withincreasing energy. Once the NPE process is driven beyond that maximumtransmittance, pulses are suppressed because lower powers experiencelower loss and are thus favored in the laser. This situation is referredto as “over-driving” the NPE.

Thus, eliminating “wavebreaking” and “over-driving” are essential inorder to achieve high pulse energy from a fiber laser. We have shownthat the first limitation, which is the more fundamental of the two, canbe avoided using self-similar pulse evolution. We have calculated theenergy of stable self-similar pulses and the result is plotted in FIG.32 as a function of net cavity dispersion. In principle, 250-nJ pulseenergies are possible, if the second limitation permits it. Thus, apromising approach is to create new saturable absorbers where“over-driving” cannot occur. In essence, we need a saturable absorber ofwhich the transmittance is not a sinusoidal function of pulse energy.Surveying the landscape of saturable absorbers used in femtosecondlasers, the real saturable absorption in a semiconductor is ideallysuited for this purpose.

Semiconductor saturable absorbers (SSA's) are based on saturation of anoptical transition, and in contrast to NPE (which is based oninterference) they cannot be overdriven. Therefore, it should bepossible to obtain much higher pulse energies in fiber lasers if NPE isreplaced by a SSA. Historically, this was not feasible, becausesemiconductor structures capable of producing the large modulation depth(>10%) needed in a fiber laser did not exist. In addition, a practicalimpediment in the past was the lack of a commercial source of suchstructures—painstaking research was required to develop new ones.However, significant progress in the modulation depth has been made inthe last several years and there is now a commercial company that sellsSSA's. BATOP GmbH (Weimar, Germany) has emerged as a reliable source ofSSA's, with a variety of designs at reasonable prices (<$1K/piece). Inparticular, structures with 80% modulation depth are available asstandard designs. It will be reasonably straightforward to incorporatethese structures in out lasers in place of NPE. The main work will beoptimizing the design of the structure for the target performancelevels.

A second major advantage of SSA's is that they are compatible withintegrated designs. The development of saturable absorbers that providefast and deep modulation will significantly facilitate the design ofall-fiber and environmentally-stable lasers. In principle, a femtosecondlaser could be constructed of segments of polarization-maintaining fiberthat provide gain and anomalous dispersion, and the saturable absorber.Fiber-pigtailed versions of SSA's are already commercially available.Such a laser would be as simple as possible, with no adjustments otherthan the pump power. We will design, construct and characterizehigh-power fiber lasers based on SSA's. Although the incorporation ofSSA with large modulation depth in a mode-locked fiber laser isrelatively new and there may be a number of practical issues to beaddressed in this work, the fundamental basis of the approach isestablished theoretically, and initial experiments in our lab withstructures from BATOP confirm that they perform as advertised. Thepromise of 25- to 50-nJ pulses directly from a robust and cost effectivefiber oscillator is highly significant. Thus, we will include thisdevelopment effort as a more exploratory component of this researchprogram, complementing our reliable (may even commercially available),but inherently more expensive, approach of a fiber CPA system.

We aim to demonstrate two all-fiber femtosecond sources with wavelengthtuning ranges of (1) 775 nm to 1000 nm and (2) 1030 nm to 1280 nm, Theoutput pulse energies will be first at 1 to 2 and then at 5 to 10 nJ. Wewill combine the femtosecond sources and the HOM fiber modules developedin Aims 1 and 2 into an all-fiber system. The fully integrated source isschematically shown in FIG. 33. The dashed boxes indicate the componentsdeveloped in Aims 1 and 2. A CPA approach for fixed wavelength fibersource is shown. SHG is needed only for the 775-nm input. The fiberlengths of the chirping fiber and the HOM fiber are approximate. Thedark dots indicate locations for fiber splicing. The cross (x) indicateslocation for fiber splicing in power tuning, or connectorization inlength or sequential tuning of multiple HOM fiber modules. The modeprofiles of the fundamental and LP₀₂ modes are also shown.

The intrinsic chirp from the fiber source 80 (either a mode-locked fiberlasers or a CPA system), which was a major limitation in previous fibersystems, provides several key advantages for our system. First, itallows ˜10 m in the total length of the output fiber. Second, the highlychirped pulse (chirp of at least 5.25 fs/nm) makes the length of thesingle mode fiber pigtail 82 inconsequential, eliminating the practicaldifficulties in cleaving and splicing. Finally, the longer single modefiber pigtail 82 can also accommodate additional fiber devices such as avariable fiber attenuator and/or a fiber optic switch.

A second harmonic generator 84 (SHG) will be employed to generatefemtosecond pulses at 775 nm. It was previously known that SHG with alinearly chirped fundamental pulse will result in a linearly chirped SHpulse, which can be subsequently compressed using linear dispersion.Interestingly, the final chirp-free SH pulse width is independent ofwhether the compression is carried out before or after the SHG 84. Theconversion efficiency, however, is obviously higher if the chirpedfundamental pulse is compressed before SHG. Because the designed pulseenergy at the fundamental wavelength is high (at 10 to 50 nJ/pulse), theconversion efficiency for SHG with the proposed excitation source willbe limited mostly by the depletion of the fundamental power, not by theavailable pulse peak intensity. Thus, SHG will be highly efficient eventwith a chirped fundamental pulse with durations on the order of severalpicoseconds if efficient doubling crystals are employed. For example,with a periodically poled LiNbO₃ (PPLN). Single-pass conversionefficiencies (energy efficiency) as much as 83% and 99% are demonstratedfor bulk and waveguide PPLN devices, respectively. Thus, SHG 84 with achirped fundamental pulse can be used with the proposed femtosecondpulse source 80 without the reduction in conversion efficiency, and, asdiscussed in the previous paragraph, has significant advantage in thesubsequent fiber optic delivery process. In addition, chirped SHG alsoeliminates the possibility of damaging to the doubling crystal due tothe high peak power of a femtosecond pulse. Photorefractive effects ofthe PPLN device is a concern at high average power (>500 mW), but sucheffects typically only occur at wavelength below 700 nm, and can bemitigated to a large extent by increasing the temperature of the crystaland/or by doping the crystal with magnesium. To be conservative, we aretargeting a power conversion efficiency of ˜50% on a routine basis.

We will design systems using two different tuning mechanisms 86: 1.power tuning, and 2. length tuning. As shown in the preliminary results,both tuning mechanisms offer similar tuning range (FIG. 28). The powertuning requires only one HOM fiber module 88 for the entire spectralrange, however, the output power varies by approximately a factor of 3(power input multiplied by the conversion efficiency). Although thispower variation across the tuning range is comparable to currentfemtosecond systems like the Ti:S or Ti:S pumped OPO, it may nonethelesslimit the practical utility of the system, particularly at the smallerwavelength shift where the output power is lowest. Another approach isfiber length tuning, which can essentially maintain the output power(FIG. 28( b), within +/−5%) across the entire spectral range. Fiberlength tuning, however, requires multiple HOM fiber modules 88,increasing the system cost. An obvious compromise is to combine the twotuning mechanisms 86. As an alternative to the power tuning, we willdesign 2 to 3 HOM fiber modules 88 of different length, each optimizedfor power tuning over a ˜100-nm spectral range to maintain a reasonablyconstant output. Such a segmented tuning also simplifies the design ofthe output LPGs 89 since a much narrower range of output wavelengthsneeds to be converted. It is interesting to note that such segmentedtuning is similar to the early generations of Ti:S lasers where multipleminor sets were required to cover the entire tuning range. However,unlike a minor-set exchange in a Ti:S laser, which would take anexperienced operator several hours to perform, the exchange of the HOMfiber modules 88 would take only a few seconds through a single modefiber connector (see the connectorized output from a fiber source inFIG. 10), and require neither experience nor knowledge of the system.For a completely electronically controlled system, a simple fiber opticswitch can be used to provide push-button HOM fiber module exchange. Infact, such a tunable HOM fiber module has already been experimentallydemonstrated several years ago for telecom applications. We also notethat, as a simple extension to the fiber length tuning, a HOM fibermodule can also be designed to provide output at the input wavelengthwithout SSFS. In such cases, the HOM fiber module simply serves as adelivery fiber for chirp compression and pulse delivery.

The fiber-length tuning described above is obviously similar to thesequential tuning described above to achieve high pulse energy. Bothrequire multiple HOM fiber modules. In length tuning, however, the sameHOM fiber of different lengths are used; while in sequential tuning, twoor more different HOM fibers are required.

Both power tuning and segmented length tuning require a mechanism tocontrol the incident power. SSFS is a nonlinear optical effect andeffectively happens instantaneously (<1 ps). Thus, the rate of thewavelength tuning of the proposed fiber source can be ultrafast, and iscompletely determined by the rate of power change. There are twoapproaches to adjust the power into the HOM fiber module. Mechanicalin-line fiber attenuators can achieve a tuning speed of ˜10 Hz, severalorders of magnitude faster than any existing laser systems. Because onlya small range of power adjustment is necessary for achieving the entirerange of wavelength tuning (less than a factor of 4 for power tuning),variable fiber attenuators that based on microbending can easily providethe speed and modulation depth required. Such a variable attenuator canbe calibrated so that rapid, electronically controlled wavelength tuningcan be achieved. We note that compact, electronically controlledvariable fiber attenuators are widely available commercially. Mostcommercial attenuators can provide modulation depth of ˜1000. Thus, wedo not anticipate any difficulty implementing the power controlmechanism. An alternative approach will be more expensive (˜$2 k), itcan easily provide nanosecond (i.e., pulse-to-pulse) wavelengthswitching speed. In addition, such a device also provides the capabilityfor fast (ns) laser intensity control. To overcome the insertion loss ofthe electro-optic modulator, it can be placed before the fiber amplifierin a CPA system. We also note that these EOMs are routinely used intelecommunications and are highly robust (telecom certified) and compact(half the size of a candy bar). Our proposed source can be readilyconfigured to provide this high speed tuning capability.

We will perform detailed system testing and characterization, providingfeedbacks for iteration and optimization of our development efforts. Inparticular, we will assess the wavelength and power stability of thesystem. We are well aware the fact that SSFS is a nonlinear opticaleffect; and nonlinear optical effects are generally sensitive tofluctuations in input power, pulse width, and pulse spectrum. We havetaken this stability issue into our design considerations. First, westart with an all-fiber, single wavelength femtosecond source. One ofthe salient features of an all-fiber design is its stability. It is wellknown that a fiber laser is more stable than a bulk solid state laser.Second, our fiber sources are specifically designed for biomedicalimaging applications. Because of the broad output pulse spectrum (10 to20 nm) and the broad excitation peaks of fluorescent molecules (tens ofnm), a few nm of wavelength shift is generally inconsequential. This isin sharp contrast to applications such as precision frequency metrology,where even a small fraction of an Angstrom spectral shift cannot betolerated. Finally, the soliton pulse shaping process is robust againstfluctuations in the input, which is one of the main reasons thatsolitons were used in long haul communication systems. Our preliminaryresults in FIG. 24 also clearly demonstrate the robustness of SSFS. Evenwith a highly nonideal input pulse (FIG. 24 inset), a nearly perfectsoliton pulse is obtained at the output. In addition, simulations with aperfect Gaussian pulse input showed good agreement with the experiments,particularly for the output at the soliton wavelength. Thus, we areconfident about the stability of the proposed source. In the unlikelyevent that unacceptably large power fluctuations are present, analternative approach is to employ feedback stabilization. Because poweradjustment mechanisms are already needed for wavelength tuning, the onlyaddition component for feedback control is a photodiode for powermonitoring (for example, through a 1% fiber tap in the single modepigtail before the LPG). Such a feedback control mechanism can largelyeliminate power drifts on the slow time scale, ˜10 Hz for the mechanicalvariable fiber attenuator and ˜MHz for the electro-optical intensitymodulator. We note that such a power stabilization scheme (“noiseeater”) has already been commercially implemented for a variety of lasersystems. We do not anticipate any difficulty implementing the controlmechanism if necessary.

Polarization control is another issue of practical concern. Forapplications that demand a linear input polarization, polarizationmaintaining (PM) fiber can be used throughout the system. Because theHOM fiber is fabricated within the conventional silica fiber platform,PM HOM fibers can be made using the same method designed forconventional PM fibers (such as adding stress rods to form a Pandafiber). For applications that demand adjustable input polarization,non-PM HOM fibers can be used and a simple in-line fiber polarizationcontroller can be used to adjust the output polarization state,eliminating the conventional free-space wave plate and/or polarizer.

There are several methods to remove the residue input light at theoutput of the HOM fiber module. Perhaps the simplest approach is todirectly deposit a dichroic coating (long wavelength pass) on the outputface of the fiber. Such coatings were often done for fiber lasers withlinear cavities and the deposition techniques were similar to that on aconventional glass substrate. After all, a silica fiber is a piece ofglass with a small diameter.

We aim to demonstrate an energetic femtosecond fiber source at 1.3 μm.The output pulse energies will be first at 2 nJ and then at greater than10 nJ. The fully integrated source 90 is schematically shown in FIG. 34.

We will fully leverage the existing technologies by using a commercial1.03 μm femtosecond fiber source 92 instead of developing our own fiberlaser. Such an approach will be most effective in cost, and will allowus to immediately focus on the HOM fibers and Cherenkov radiation. Anumber of companies, such as PolarOynx Inc (photograph of device shownin FIG. 34) and IMRS America, already provide sources that perfectly fitour applications. After propagating through the HOM fiber module 94, theoutput will be spectrally filtered to remove light outside the 1.24 to1.3 μm bandwidth. Perhaps the simplest approach for optical filtering isto directly deposit a dichroic coating (long wavelength pass) on theoutput face of the fiber. Such coatings were often done for fiber laserswith linear cavities and the deposition techniques were similar to thaton a conventional glass substrate. Dispersion compensation 96 will beused to remove the linear chirp of the Cherenkov radiation so that theshortest pulses can be obtained at the output. The amount of dispersionneeded (˜0.01 ps/nm) for de-chirping the Cherenkov radiation isequivalent to that of several centimeters of HOM fiber (after all, 25 cmor less HOM is used to generate the Cherenkov). Such a small amount ofdispersion can be provided by a variety of methods. For example, prismpairs 98 made of pure silicon or dense glass (schematically shown inFIG. 34), which are low loss, low cost, and easy to operate, can readilyprovide the necessary dispersion value. As discussed in the previoussection, for Cherenkov radiation above 10 nJ, we also have the option toeliminate the dispersion compensation after the HOM if a slightly longerpulse width (100 to 200 fs) is acceptable.

We will perform detailed system testing and characterization, providingfeedbacks for iteration and optimization of our development efforts. Inparticular, we will assess the wavelength and power stability of thesystem. We are well aware of the fact that Cherenkov radiation is anonlinear optical effect; and nonlinear optical effects are generallysensitive to fluctuations in input power, pulse width, and pulsespectrum. We have taken this stability issue into our designconsiderations. First, we start with a fiber femtosecond source. One ofthe salient features of a fiber source is its stability. It is wellknown that a fiber laser is more stable than a bull solid state laser.Second, our fiber sources are specifically designed for biomedicalimaging applications. Because of the broad output pulse spectrum and thebroad excitation peaks of fluorescent molecules (tens of nm), a few nmof wavelength shift is generally inconsequential. This is in sharpcontrast to applications such as precision frequency metrology, whereeven a small fraction of an Angstrom spectral shift cannot be tolerated.Finally, the soliton pulse shaping and Cherenkov radiation is robustagainst fluctuations in the input, as demonstrated by both our numericalsimulations and previous experiments.

Polarization control is another issue of practical concern. Forapplications that demand a linear input polarization, polarizationmaintaining (PM) fibers can be used throughout the system. Because theHOM fiber is fabricated within the conventional silica fiber platform,PM HOM fibers can be made using the same method designed forconventional PM fibers (such as adding stress rods to form a Pandafiber). For applications that demand adjustable input polarization,non-PM fibers can be used and a simple in-line fiber polarizationcontroller can be used to adjust the output polarization state,eliminating the conventional free-space wave plate and/or polarizer.

We will demonstrate the significance of our new femtosecond sources forbiomedical applications of multiphoton microscopy, spectroscopy andendoscopy. Cornell's strong biomedical nonlinear imaging researchinfrastructure provides an ideal proving ground for new femtosecondsources and our proposed laser development is highly synergistic withthese existing research programs. Cornell is home to the DevelopmentResource for Biophysical Imaging & Optoelectronics (DRBIO), anNIH-funded Resource, where multiphoton microscopy was originallydeveloped. A number of multiphoton microscopes currently exist in thelaboratories of both the PI and DRBIO.

The proposed longer wavelength femtosecond source offers unprecedentedcapability at the 1.3 μm wavelength window. Although there are only afew experimental demonstrations for multiphoton imaging beyond 1.1 μm,longer wavelength multiphoton imaging is feasible and can potentiallyoffer significant advantage in deep tissue imaging, particularly withthe high pulse energy we will be able to obtain. As a part of the NSFCAREER grant for deep tissue imaging, we have already started our efforton exploring this new spectral window for MPM, using the existing Ti:Spumped OPO at the PI's lab. We will demonstrate the capability of theproposed femtosecond source for imaging in the 1300 nm region usingindicators such as shown in FIG. 29 and IR quantum dots. We will comparethese results with what we currently achieve using the OPO system. Weaim to achieve unprecedented imaging depth using the energetic pulsesfrom our source. We note that these proof-of-concept experiments are notintended for answering specific biological questions. Our main objectiveis to compare the imaging performance of MPM with the proposed sourcesand with conventional femtosecond lasers. These validation experiments,carried out in a real-world biomedical imaging facility, are essentialto establish the value of the proposed sources. We believe that thecreation of an energetic femtosecond fiber source at the wavelengthwindow of 1300 nm will open significant new opportunities for deeptissue imaging.

Our first stage demonstration involves “routine” multiphoton imaging andspectroscopy. We will compare the capability of the proposed tunablefiber source with our existing Ti:S lasers. We will verify the stabilityof the sources in imaging, especially since the two (and three)photon-dependence of excitation “amplifies” the effects of a fluctuatinglaser. In addition to multiphoton imaging, a potentially even moresensitive means to judge stability would be to test the laser as anexcitation source in fluorescence correlation spectroscopy (FCS)experiments, where laser noise (e.g., oscillations) would be veryobvious (e.g., FCS measurements on the same sample made with our Ti:Scompared to ones carried out with our prototype laser). By installingour prototype laser source on one or more multiphoton systems we willtest the laser in the most practical way by using it in a routineday-to-day fashion for a variety of imaging projects. Our main objectiveis to compare the imaging performance of MPM with the proposed sourcesand with conventional Ti:S lasers.

Our second stage demonstration experiments are designed to showcase theunique advantages of the proposed femtosecond sources, which haveseveral important functional attributes for multiphoton imaging notfound in the commonly used Ti:S laser. A review of the properties of thelasers being developed and their importance to multiphoton imaginginclude: (1) all-fiber sources with integrated fiber delivery, (2)rapid, electronically controlled wavelength tuning, and (3) energeticpulses, particularly at the longer wavelength window of 1030 to 1280 nm.

The two femtosecond sources proposed are both all-fiber sources withintegrated single mode fiber-delivered (with >5 m of fiber length); thatis, the output of the sources could be directly fed into a microscopescanbox, an endoscope scanning system, or through a biopsy needle fortissue spectroscopy. For multiphoton microscopy this would greatlysimplify installation and maintenance of the system since alignmentwould be trivial; and for endoscopic imaging and spectroscopyapplication fiber-delivered illumination is clearly essential.

A stable fiber-delivered femtosecond source in the 780-850 nm range canbe directly incorporated in our current endoscope scanner design, asshown in FIG. 35. We also note that the HOM fibers are highly resistantto bending loss, a characteristic that is impossible to obtain in thelarge area mode fiber previously demonstrated for pulse delivery. Thus,it is particularly suited for small diameter, flexible endoscopes wherebend radius as small as ˜1 cm is necessary. Although no clinicalexperiment is planned within the scope of this program, the long fiberdelivery length (˜6 m) allows the source to be at a remote location awayfrom the operating room. In a clinical environment, such a physicalseparation offers major practical advantages, such as eliminating thecomplications of sterilization, ultimately leading to a much reducedcost.

As shown in FIG. 35, HOM fiber 91 that provides the dispersioncompensation and wavelength tuning through SSFS can also besimultaneously used as the delivery and collection fiber for tissuespectroscopy. The diameter of the optical fiber 91 is ˜0.125 mm(standard size for single mode fiber), which is much smaller than theinside diameter of an 18 or 20 gauge needle that is routinely used forcore biopsy. The excited signal will be collected by the same fiber 91.A fiber wavelength division multiplexer (WDM) 93 can be placed betweenthe fixed wavelength femtosecond source 95 and the HOM fiber module 91to direct the collected signal to the detecting unit 97, which consistsa grating and a CCD. In addition, the rapid wavelength tuning capabilityallows the emission spectrum of the tissue to be recorded as a functionof the excitation wavelength. These multiphoton excited fluorescenceexcitation-emission matrix (EEM) can potentially provide uniquediagnostic signatures for cancer detection just as one-photon EEM does.The long delivery fiber (HOM fiber) once again allows the excitation anddetection apparatus to be at locations away from the operating room. Wefurther note that a double-clad fiber structure with the HOM fiber asthe guiding core can easily fabricated to improve signal collectionefficiency because of the all-silica fiber design.

One potential complication of the proposed tunable source formultiphoton EEM is the power and pulse width variation across the entiretuning range. Calibration using a known multiphoton excitation standard,such as fluorscein dye, will be carried out before experimentation onbiological samples. Such a calibration procedure is routinely used inprevious multiphoton spectroscopy work. Multiphoton excitation standardshave been established in the past and has extensive experience inmultiphoton spectroscopy. We don't expect significant problems in thecalibration of the instrument.

Application of MPM in early cancer detection using a transgenic mouseline in which tumor formation is initiated by the conditional activationof the p53 and Rb1 genes by Adenovirus-Cre-mediated recombination hasbeen reported. The experiment on endoscopes and tissue spectroscopythrough needle biopsies is highly synergistic with the on-going cancerresearch and provides an ideal platform for showcasing the “all-fiber”characteristics of the proposed femtosecond source.

A unique capability of the proposed sources is the ability to rapidlytune the wavelength much faster than currently possible with single boxTi:S systems. Rapid wavelength tuning would allow for line by lineswitching between excitation wavelengths during scanning, or forcollecting excitation spectra, a potentially important parameter forbiomedical applications that may utilize intrinsic fluorophores withoverlapping emissions, but different excitation spectra.

By synchronizing the wavelength control with the scanning andacquisition, we will modify one of our imaging systems to enable onewavelength during the “forward” line and a second during the return(without changing the Y position). This is analogous to what is nowstandard on modern AOM-equipped confocal microscopes, where, forexample, a green dye is excited with 488 nm excitation in one directionand 547 nm excitation to excite a different dye during the return. Inthis way a two-color image can be collected using dyes with differentexcitation maximums and separable emissions. The temporal aspecteliminates problems with spectral cross-talk in many cases. Althoughmultiphoton cross-sections for many dyes are broad often allowing forexcitation of different dyes at the same wavelength (usually due tooverlapping UV bands, so this normally only works at 800 nm or shorter),the ability to rapidly switch between wavelengths anywhere between 780and 1000 nm would be an important enhancement for many dye pairs. Afterinterfacing the wavelength control with our scanning systems we willapply this capability in pilot experiments with fluorophores such as CFPand GFP which have different two photon excitation maxima, but partiallyoverlapping emission spectra (FIG. 36).

As an added benefit, the EOM device that enables rapid wavelength tuningcan also be used to provide fast switching and modulation of theexcitation beam. At a minimum this functionality should be comparable towhat we currently achieve using our 80-nm resonance-dampened KTP* Pockelcells for routing beam blanking and intensity control (microsecondswitching). Available fiber-coupled EOMs can switch in thesub-nanosecond range and should allow for a laser with a built-inmodulator that would enable the user to reduce the effective laserrepetition rate for measurements of fluorescent decay times andfluorescent lifetime imaging (FLIM), as well as for the more standardmodulation needs. After implementing the required control electronics,we will use this functionality for routine beam blanking and control,photobleaching recovery measurements, and FLIM.

Another intriguing possibility provided by SSFS is that multiplewavelength tunable pulses can be obtained from the same fixed wavelengthfiber source. For example, the output of the fixed wavelengthfemtosecond fiber source can be split into two halves and each halfpropagates through a HOM fiber module. The two HOM fiber modules can bethe same (use power tuning) or of different lengths (length tuning).Such a multi-color femtosecond source opens a range of newopportunities, such as two-color photon excitation and coherentanti-Stokes Raman scattering (CARS) imaging, where two synchronizedultrafast sources are needed previously. The spectral bandwidthsdirectly from the proposed sources will likely be too large for CARS,possibly requiring spectral filtering or shaping.

The proposed longer wavelength femtosecond source offers unprecedentedcapability at the wavelength window of 1030 to 1280 nm. Although thereare only a few experimental works for multiphoton imaging beyond 1100nm, longer wavelength multiphoton imaging is feasible and canpotentially offer significant advantage in deep tissue imaging,particularly with the high pulse energy we will be able to obtain.Efforts are underway on exploring this new spectral window for MPM,using the existing Ti:S pumped OPO. We will demonstrate the capabilityof the proposed femtosecond source for imaging in the 1.27 μm regionusing indicators such as shown in FIG. 29 and IR quantum dots. We willcompare these results with what we currently achieve using the OPOsystem. We aim to achieve unprecedented imaging depth using theenergetic pulses from our source. There is no doubt that the creation ofan all-fiber, wavelength tunable, energetic femtosecond source at thelonger wavelength window of 1030 to 1280 nm will open significant newopportunities for biomedical imaging.

Although preferred embodiments have been depicted and described indetail herein, it will be apparent to those skilled in the relevant artthat various modifications, additions, substitutions and the like can bemade without departing from the spirit of the invention and these aretherefore considered to be within the scope of the invention as definedin the claims which follow.

1. A method of producing optical pulses having a desired wavelength ofless than 1300 nm, said method comprising: generating input opticalpulses using an optical pulse source, wherein the input optical pulseshave a first wavelength, a first spatial mode and exhibit a linear chirpof at least 5.25 fs/nm; shifting the mode of the input optical pulses toa second, higher-order mode; and delivering the mode-shifted inputoptical pulses into a higher-order-mode (HOM) fiber configured tosupport the propagation of the second, higher-order mode of the inputoptical pulses and configured to alter the wavelength thereof to adesired, second wavelength, the desired, second wavelength being alonger wavelength than the first wavelength and having a value of lessthan 1300 nm.
 2. The method as defined in claim 1 wherein the wavelengthis altered in the HOM fiber by soliton self-frequency shifting (SSFS).3. The method as defined in claim 1 wherein Cherenkov radiation altersthe wavelength of the input optical pulses to generate output pulses atthe desired, second and longer wavelength within the HOM fiber module,thereby producing output optical pulses having the desired wavelength.4. The method according to claim 1, wherein the HOM fiber is a solidsilica-based fiber.
 5. The method according to claim 1 furthercomprising: reconverting the second spatial mode of the output opticalpulses back to the first spatial mode.
 6. The method according to claim1, wherein the optical pulse source generates input optical pulseshaving an energy of at least 1.0 nanojoules (nJ).
 7. The methodaccording to claim 1, wherein the optical pulses source generates inputoptical pulses having a pulse energy of between 1.0 nJ and about 100 nJ.8. The method according to claim 1 further comprising: tuning the firstwavelength of the input optical pulses to an intermediate wavelengthprior to delivering the input optical pulses into the HOM fiber.
 9. Themethod according to claim 8, wherein the tuning comprises subnanosecondpower tuning using a power control system connectedly disposed betweenthe optical pulse source and the HOM fiber.
 10. The method according toclaim 1 further comprising: varying a length of the HOM fiber so as tovary the desired wavelength.
 11. The method according to claim 1 furthercomprising: varying a power of the input optical pulses so as to varythe desired wavelength.